Theatre backing erstwhile actor, say (9)
I believe the answer is:
performer
'actor say' is the definition.
(actor is a kind of performer)
'theatre backing erstwhile' is the wordplay.
'theatre' becomes 'rep' (repertory theatre).
'backing' shows that the letters should be reversed in order.
'erstwhile' becomes 'former' (synonyms).
'rep' backwards is 'per'.
'per'+'former'='PERFORMER'
(Other definitions for performer that I've seen before include "star?" , "clown possibly" , "Entertainer" , "Eg 9 [ACTRESS]" , "artist" .)