400 years ago English was quite different. Defoe published Robinson Crusoe a year before the Pilgrims landed on Plymouth Rock, for comparison.
One of the things about the conjunction but is that it presupposes some surprise; i.e, there is always some expectation the speaker would normally make that is contradicted by the facts that follow but.
- He's very smart, but not about social things. (presupposing consistency of intelligence)
- It's sposta be here by 11, but it's late today. (presupposing normal schedule)
This comes from the earlier use of but to mean only, a negative trigger, in the second example.
- my arable land was but small, suited to my crop
Nowadays this use is rare, but 400 years ago it was apparently quite common, at least in writing. In Modern English we'd most likely float the only, and use but just to mark the presupposition that more arable land was needed:
- my only arable land was small, but [it was] suited to [the (small) size of] my crop
That's the second example. The first example is a Nobbut-Cleft. Consider what he could have said, but didn't:
- I solved the problem by making an enclosure with a hedge.
Commendably clear, but not cool (using 17th-century notions of what's cool). So he expands on it by clefting it. This is a Cleft sentence:
- The only way I could see to do it was by making an enclosure with a hedge.
Note the was -- Cleft sentences often insert some form of be or some other There-Insertable verb as the fulcrum for the cleavage, then isolate the topic before the be and the comment after it.
This is a Pseudo-Cleft (or Wh-Cleft) sentence, which also inserts be and marks the topic with a Wh-word:
- What I did about it was (to) make an enclosure with a hedge.
There are other varieties of clefting in English, and the Nobbut-Cleft is one:
- There remains no more but to thank you for your courteous attention.
There is always a negation in the clause preceding but, and the but clause illustrates an exception to the negation.
The third example does not appear to represent what Defoe meant in the second example, so I won't treat it here, except to say that it is indeed, grammatical, though it doesn't make sense in context.