Refuse to study unknown (4)
I believe the answer is:
deny
'refuse' is the definition.
(I've seen this before)
'study unknown' is the wordplay.
'study' becomes 'den' (I've seen this before).
'unknown' becomes 'y' (algebra).
'den'+'y'='DENY'
'to' is the link.
(Other definitions for deny that I've seen before include "Declare non-existent" , "Refuse access to" , "Disclaim" , "Reject as false" , "Refuse to acknowledge" .)