Line of waiters, we hear, may be prompt (3)
I believe the answer is:
cue
'be prompt' is the definition.
(cue can mean a prompt or signal)
'line of waiters we hear' is the wordplay.
'line of waiters' becomes 'queue' (I've seen this in another clue).
'we hear' shows a homophone (sound like).
'queue' is a homophone of 'CUE'.
'may' is the link.
(Other definitions for cue that I've seen before include "Signal (to actor)" , "signal to come on" , "Snooker rod" , "Prompt; reminder" , "On a car[D?] this would give a prompt" .)