Study, one taken by a pupil in opposition (6)
I believe the answer is:
denial
'opposition' is the definition.
(thesaurus)
'study one taken by a pupil' is the wordplay.
'study' becomes 'den' (term for a study or office).
'one' becomes 'i' (Roman numeral).
'taken by' is a charade indicator (letters next to each other).
'pupil' becomes 'L' (as in L-plates for a learner driver).
'den'+'i'+'a'+'l'='DENIAL'
'in' is the link.
(Other definitions for denial that I've seen before include "Declaration that something is untrue" , "Nailed (anag.)" , "Saying it isn't so" , "Refusal, abnegation" , "Negation" .)