Oppose son entering repeat exam (6)
I believe the answer is:
resist
'oppose' is the definition.
(resisting is a kind of opposing)
'son entering repeat exam' is the wordplay.
'son' becomes 's'.
'entering' indicates putting letters inside.
'repeat exam' becomes 'resit'.
's' placed into 'resit' is 'RESIST'.
(Other definitions for resist that I've seen before include "Express opposition" , "Refuse to comply" , "Sister (anag)" , "Go against" , "Oppose, put up defence" .)