They set off to get the books (7)
I believe the answer is:
primers
'books' is the definition.
(primer is a kind of book)
'they set off to get' is the wordplay.
I cannot really see how this works, but
'set' could be 'prime' (priming is a kind of setting) and 'prime' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'the' acts as a link.
Can you help me to learn more?
(Other definitions for primers that I've seen before include "detonators" , "Introductory books or undercoats" .)