Not entirely prepared to study (4)
I believe the answer is:
read
'study' is the definition.
('read' can be a synonym of 'study')
'not entirely prepared' is the wordplay.
'not entirely' means to remove the last letter.
'prepared' becomes 'ready' ('ready' can be similar in meaning to 'prepared').
'ready' with its last letter taken away is 'READ'.
'to' is the link.
(Other definitions for read that I've seen before include "examined" , "Interpreted, for example" , "Lecture" , "Use a book" , "Undertake a course of study" .)