Signal given by letter audibly (3)
I believe the answer is:
cue
'signal' is the definition.
(as in an actor's cue)
'letter audibly' is the wordplay.
'letter' becomes 'q'.
'audibly' indicates a 'sounds like' (homophone) clue.
'q' sounds like 'CUE'.
'given by' is the link.
(Other definitions for cue that I've seen before include "Actor's signal" , "signal to come on" , "Prompt; signal" , "Billiards stick" , "line in speech" .)