Group of waiters said to be prompt (3)
I believe the answer is:
cue
'be prompt' is the definition.
(cue can mean a prompt or signal)
'group of waiters said' is the wordplay.
'group of waiters' becomes 'queue' (people waiting in a line).
'said' indicates a 'sounds like' (homophone) clue.
'queue' sounds like 'CUE'.
'to' is the link.
(Other definitions for cue that I've seen before include "Sporting rod" , "line in speech" , "Prompting" , "Actor's words" , "Actor's prompt" .)