Those waiting to hear a signal (3)
I believe the answer is:
cue
'a signal' is the definition.
(I've seen this before)
'those waiting to hear' is the wordplay.
'those waiting' becomes 'queue'.
'to hear' shows a homophone (sound like).
'queue' sounds like 'CUE'.
(Other definitions for cue that I've seen before include "Hint on stage" , "Billiard stick - hint - signal" , "Potter" , "Prompt; reminder" , "Hint, prompt" .)