Not fully prepared to study (4)
I believe the answer is:
read
'study' is the definition.
('read' can be a synonym of 'study')
'not fully prepared' is the wordplay.
'not fully' means to remove the last letter.
'prepared' becomes 'ready' ('ready' can be similar in meaning to 'prepared').
'ready' with its last letter taken off is 'READ'.
'to' acts as a link.
(Other definitions for read that I've seen before include "Study (for a degree?)" , "what follows 18 [OMASUM] in a 17 [RUMINANT]" , "Interpret (text)" , "Absorbed (written text)" , "Use a book" .)