Theatre upset erstwhile actor (9)
I believe the answer is:
performer
'actor' is the definition.
(actor is a type of performer)
'theatre upset erstwhile' is the wordplay.
'theatre' becomes 'rep' (repertory theatre).
'upset' says the letters should be written backwards.
'erstwhile' becomes 'former' (synonyms).
'rep' back-to-front is 'per'.
'per'+'former'='PERFORMER'
(Other definitions for performer that I've seen before include "Such as a musician or actor" , "player" , "artist" , "star?" , "Entertainer" .)