Signal to line up for hearing (3)
I believe the answer is:
cue
'signal' is the definition.
(as in an actor's cue)
'line up for hearing' is the wordplay.
'line up' becomes 'queue' (synonyms).
'for hearing' indicates a 'sounds like' (homophone) clue.
'queue' sounds like 'CUE'.
'to' is the link.
(Other definitions for cue that I've seen before include "Hint on stage" , "Rod used in billiards" , "Prompt; signal" , "Actor's prompt" , "Used in snooker and on stage" .)