Defiant chap entering further exam (9)
I believe the answer is:
resistant
'defiant' is the definition.
(similar in meaning)
'chap entering further exam' is the wordplay.
'chap' becomes 'Stan'.
'entering' is an insertion indicator.
'further exam' becomes 'resit' (I've seen this before).
'stan' placed into 'resit' is 'RESISTANT'.
(Other definitions for resistant that I've seen before include "Withstanding adverse conditions" , "not accommodating" , "Hindering the conductor" , "Not affected by something, a bug by an antibiotic say" , "Able to withstand adverse conditions" .)