Minor member of cast often subject to examination (10)
I believe the answer is:
understudy
'minor member of cast often' is the definition.
I can't tell whether this defines the answer.
'subject to examination' is the wordplay.
'subject to' becomes 'under' (being subject to a law is being under it).
'examination' becomes 'study' (study is a kind of examination).
'under'+'study'='UNDERSTUDY'
Can you help me to learn more?
(Other definitions for understudy that I've seen before include "one covers part" , "Replacement" , "emergency role" , "One prepared to take over from leading actor" , "reserve?" .)