In the two-word phrase for sure, the word for is a preposition and the word sure is its complement, even though it is an adjective not a noun. (An argument that it is a conjunction here not a preposition is possible, but I shall not be making it.)
Specifically, this is the sense of for that means as or as being. Notice that as being lends itself to accepting complements that are equally likely to be nouns or adjectives.
The OED explains in sense 19b below:
for, prep. and conj.
A. prep.
VI. Of attributed or assumed character; = as.
- a. In the character of, in the light of, as equivalent to; esp. to
introduce the complement after verbs of incomplete predication, e.g. to
have, hold, etc. (see those verbs), where as or as being may generally
be substituted. to beg (a person) for a fool: see beg v. 5a.
b. So with an adjective, as in to take for granted, to leave for
dead, etc. for certain, for sure, †for wiss, see those adjectives.
Also, with mixture of sense A. 8, as in the formula of the Marriage
Service (quot. 1549) where the sense is ‘whether she prove better or
worse’, etc.
And under sure adj. they give this sense for that phrase down in the phrases section:
sure, adj., adv., and int.
Phrases
P4. for sure: as a certainty, for certain; without doubt; undoubtedly. Frequently in that’s for sure. In later, colloquial use often used emphatically at the end of a sentence.
As syntactic constituents, prepositional phrases are always modifier phrases; they are neither noun phrases nor verb phrases. Your phrase for sure is no different than any other prepositional phrase in this regard. That means you can exchange it for any other syntactic constituent acting in a modifier role.
Consider this citation, a fairly typical use:
1971 C. Bonington Annapurna South Face xiii. 156
We can't do it in the next two days..—that's for sure.
Here for sure is acting as a predicate complement to that is. If you swapped it for the adjective certain, it would be perfectly grammatical and indeed mean the very same thing. Because that would be a predicate adjective, that means here your prepositional phrase is acting as an adjectival modifier phrase, not as an adverbial one.
But consider how it is used in this earlier but better known citation:
1883 Robert Louis Stevenson Treasure Island ɪ. vi. 52
These fellows who attacked the inn tonight—bold, desperate blades, for sure.
Here I would not attempt to make the case that your prepositional phrase is being used adjectivally. Notice that you could exchange it for either of surely or certainly, both adverbs, which no change in grammar or indeed meaning. That makes it an adverbial modifier — although I leave it to others to decide whether it’s acting on the adjectives bold, desperate, or whether it’s actually a sentence adverbial against an entire copular clause whose verb has now gone missing (surely is well known for being able to be a sentence adverb).
There’s one other case to look at, though. Instead of predicative uses, it can also be used attributively. Here are several illustrations from Google Books:
- With the microchip, it is not a for sure bet, but your odds are much higher of getting your lost dog back, then [sic] if you did not have it.
- Pick a date of the month and make it a “for sure” time to get together and discuss money.
- It looked like a dragon, kind of, but it wasn't a “for sure” sort of thing. What was for sure was that it was adorable.
- This is accomplished by not giving a "for sure" option at all (Table 5, Situation 2). If the subject is not indifferent to the two options in Situation 2, adjustment is allowed in order to guard against the "for sure" bias.
Those attributive uses just given are all clearly adjectival ones because you can exchange them for the adjective certain without affecting the grammar at all, nor even the meaning.
Summary
The prepositional phrase for sure comprises the preposition for and its complement sure, which is an adjective. It remains a prepositional phrase no matter what use you put it to.
It is not possible to categorically state that the prepositional phrase for sure is always acting adverbially because that assessment can be made only within a surrounding syntactic context. Sometimes it is an adverbial modifier phrase and sometimes it is an adjectival modifier phrase. You can determine which applies by swapping it for a single-word adjective or a single-word adverb.
