Signal letter read out (3)
I believe the answer is:
cue
'signal' is the definition.
(thesaurus)
'letter read out' is the wordplay.
'letter' becomes 'q'.
'read out' indicates a 'sounds like' (homophone) clue.
'q' sounds like 'CUE'.
(Other definitions for cue that I've seen before include "Snooker player's stick" , "Used in snooker and on stage" , "Actor's prompt" , "Sign" , "Signal for player" .)