The answers so far are oversimplified, arguably to the point of being plain wrong.
What you are proposing here is called notional concord:
As Quirk et al. 1985 explains it, notional agreement (called notional concord by Quirk and others) is agreement of a verb with its subject or of a pronoun with its antedecent in accordance with the notion of number rather than with the presence of an overt grammatical marker for that notion. Another way to look at the matter is that of Roberts 1954, who explains that notional agreement is agreement based on meaning rather than form.
The corresponding Wikipedia entry is synesis:
Synesis [...] is effectively an agreement of words with the sense, instead of the morphosyntactic form. [...] Such use in English grammar is often called notional agreement (or notional concord), because the agreement is with the notion of what the noun means, rather than the strict grammatical form of the noun (the normative formal agreement). The term situational agreement is also found[.]
Notional agreement for collective nouns is very common in British English. It is less customary in American English, but may sometimes be found after phrases of the type "a collective noun of plural nouns"[.]
That bears repeating. Notional concord is quite common in English:
- A lot of people are.
- A handful of people are.
- A majority of people are.
- A number of cars are.
- A variety of species are.
- A multitude of elements are.
- A few folks are.
- A couple cats are.
- A total of seven students are.
Conversely, you can have things such as "lots/piles/truckloads of money is".
So, the question is not whether notional agreement is a thing. The question is, whether it is a thing in this particular construction you are looking at, "a combination of [Xs]".
And there is no way at all to answer that question on a theoretical level, without looking at what native speakers actually say and write. Any answer that does not look at the reality of the language is not an answer, but mere opinion.
So, after this long-drawn-out preamble, let us look at the reality of the language, then. Here are the actual usage stats from:
From these you can clearly see two things:
- In American English, notional agreement in this particular construction is possible, and common. However, it is less common than singular concord.
- In British English, notional agreement in this construction is unheard of.
In conclusion, if you want to be on the safe side, use the singular.