A cry of surprise about theatre’s foremost performer (5)
I believe the answer is:
actor
'performer' is the definition.
(actor is a kind of performer)
'a cry of surprise about theatre's foremost' is the wordplay.
'cry of surprise' becomes 'cor' (colloquial).
'about' means one lot of letters goes inside another.
'foremost' says to take the initial letters.
The first letter of 'theatres' is 't'.
'cor' going around 't' is 'ctor'.
'a'+'ctor'='ACTOR'
(Other definitions for actor that I've seen before include "Gielgud, say" , "Stage-player" , "Croat (anag.)" , "Films/stage worker" , "One with role" .)