Match being held inside cannot upset competitor (10)
I believe the answer is:
contestant
'competitor' is the definition.
(someone competing in a game)
'match being held inside cannot upset' is the wordplay.
'match' becomes 'test' (test match is a kind of match).
'being held inside' means one lot of letters goes inside another.
'upset' is an anagram indicator.
'cannot' is an anagram of 'conant'.
'test' put into 'conant' is 'CONTESTANT'.
(Other definitions for contestant that I've seen before include "Candidate" , "One of those competing" , "One in quiz" , "One entering a competition" , "Competitor" .)