Refuse to accept study by unknown (4)
I believe the answer is:
deny
'refuse to accept' is the definition.
(I've seen this before)
'study by unknown' is the wordplay.
'study' becomes 'den' (I've seen this before).
'by' means one lot of letters go next to another.
'unknown' becomes 'y' (algebra).
'den'+'y'='DENY'
(Other definitions for deny that I've seen before include "Reject as true" , "Disown" , "Declare untrue" , "Refuse to give" , "contest" .)