Study unknown refuse (4)
I believe the answer is:
deny
'refuse' is the definition.
(I know that refuse can be written as deny)
'study unknown' is the wordplay.
'study' becomes 'den' (den can mean an office or study).
'unknown' becomes 'y' (algebra).
'den'+'y'='DENY'
(Other definitions for deny that I've seen before include "Refuse to admit the truth" , "Declare not to be true" , "Gainsay" , "Repudiate, disavow" , "Declare untrue" .)