Letter read out prompt (3)
I believe the answer is:
cue
'prompt' is the definition.
('cue' can be a synonym of 'prompt')
'letter read out' is the wordplay.
'letter' becomes 'q'.
'read out' shows a homophone (sound like).
'q' is a homophone of 'CUE'.
(Other definitions for cue that I've seen before include "Actor's line to remind another" , "Sporting rod" , "Actor's signal" , "Prompt; reminder" , "Hint for a snooker or billiards striker" .)