Not admit study takes a year (4)
I believe the answer is:
deny
'not admit' is the definition.
('deny' is the opposite to 'admit')
'study takes a year' is the wordplay.
'study' becomes 'den' (I've seen this before**).
'takes' means one lot of letters go next to another.
'a year' becomes 'y'.
'den'+'y'='DENY'
(Other definitions for deny that I've seen before include "Repudiate, disavow" , "Forbid; contradict" , "Negate" , "Declare untrue" , "Refuse to admit (something)" .)