Signal for an actor (3)
I believe the answer is:
cue
'signal for an actor' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for cue that I've seen before include "Hint for a snooker or billiards striker" , "Hint, prompt" , "Sign" , "Prompt; signal" , "line in speech" .)