Player’s mascot, poor at intervals (5)
I believe the answer is:
actor
'player's' is the definition.
(synonyms)
'mascot poor at intervals' is the wordplay.
'at intervals' means one should take alternating letters (letters are taken at regular intervals).
'mascot'+'poor'='mascotpoor'
The alternating letters of 'mascotpoor' are 'ACTOR'.
(Other definitions for actor that I've seen before include "Theatre player" , "Theatrical performer" , "For example, Colman" , "ham" , "One on stage" .)