Prompt letter, by the sound of it? (3)
I believe the answer is:
cue
'prompt' is the definition.
(cue can mean a prompt or signal)
'letter by the sound of it?' is the wordplay.
'letter' becomes 'q'.
'by the sound of it?' indicates a 'sounds like' (homophone) clue.
'q' is a homophone of 'CUE'.
(Other definitions for cue that I've seen before include "Tapering stick used by snooker player" , "Prompt; signal" , "Prompting" , "Billiard stick - hint - signal" , "Signal for action" .)