He's a gold medallist (6)
I believe the answer is:
victor
'medallist' is the definition.
The answer and definition can be both people as well as being singular nouns.
Maybe you can see an association between them that I can't see?
'he's a gold' is the wordplay.
'he' becomes 'Vic' (short for Victor).
'a gold' becomes 'tor' (I can't justify this - if you can you should believe this answer much more).
'vic'+'tor'='VICTOR'
Can you help me to learn more?
(Other definitions for victor that I've seen before include "Competitor who's first" , "winning player" , "Oh dear!" , "''To the . . . . . . the spoils'' (6)" , "Winner in a contest" .)