Runner gutted about expert competitor (5)
I believe the answer is:
racer
'competitor' is the definition.
(someone competing in a race)
'runner gutted about expert' is the wordplay.
'gutted' says to hollow out the word (remove centre letters) (to gut something is to remove its insides).
'about' indicates putting letters inside.
'expert' becomes 'ace' (ace can informally mean very good).
'runner' with its centre removed is 'rr'.
'rr' enclosing 'ace' is 'RACER'.
(Other definitions for racer that I've seen before include "Runner" , "Fast competitive car or animal" , "Participant in speed contest" , "Bike, perhaps" , "someone driving very fast?" .)