Book studied to make money (5)
I believe the answer is:
bread
'money' is the definition.
(bread is a kind of money)
'book studied' is the wordplay.
'book' becomes 'b'.
'studied' becomes 'read' (I've seen this before).
'b'+'read'='BREAD'
'to make' is the link.
(Other definitions for bread that I've seen before include "Gracefully thin" , "Money (inf)" , "Staple diet" , "Eg, pumpernickel" , "Dosh (slang)" .)