A surprised comment about debut of theatre performer (5)
I believe the answer is:
actor
'performer' is the definition.
(actor is a kind of performer)
'a surprised comment about debut of theatre' is the wordplay.
'surprised comment' becomes 'cor' (exclamation of surprise).
'about' means one lot of letters goes inside another.
'debut of' suggests taking the first letters (I've seen 'debut' mean this).
The first letter of 'theatre' is 't'.
'cor' enclosing 't' is 'ctor'.
'a'+'ctor'='ACTOR'
(Other definitions for actor that I've seen before include "One on stage" , "One in a play" , "Cruise, e.g." , "Films/stage worker" , "Player - performer" .)