Study unknown counter (4)
I believe the answer is:
deny
'counter' is the definition.
(thesaurus)
'study unknown' is the wordplay.
'study' becomes 'den' (term for a study or office).
'unknown' becomes 'y' (algebra).
'den'+'y'='DENY'
(Other definitions for deny that I've seen before include "Gainsay" , "Refute, contradict" , "Refuse to admit the truth" , "Disclaim" , "Declare non-existent" .)