Competitor, presumably not an outsider (7)
I believe the answer is:
entrant
'competitor' is the definition.
(someone who has entered a competition)
'presumably not an outsider' is the wordplay.
I cannot really understand how this works, but
'not' could be 'nt' (synonyms) and 'nt' is found in the answer.
'an' could be 'a' and 'a' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for entrant that I've seen before include "Contestant; starter" , "Someone signing up for a competition" , "Participant in, eg, a race" , "Newcomer" , "Competitor, contestant" .)