Pay for courses, with books in principle (11)
I believe the answer is:
fundamental
'books in principle' is the definition.
'fundamental' can be an answer for 'principle' (fundamental principle is a kind of principle). I'm not certain of the 'books in' bit.
'pay for courses' is the wordplay.
'pay for' becomes 'fund' (synonyms).
'courses' becomes 'amental' (I can't explain this - if you can you should give a lot more credence to this answer).
'fund'+'amental'='FUNDAMENTAL'
'with' acts as a link.
Can you help me to learn more?
(Other definitions for fundamental that I've seen before include "Of central importance" , "key" , "Basic" , "Primary" .)