Signal heard to unexpectedly attack and gain an unfair advantage (5-4)
I believe the answer is:
queue-jump
'gain an unfair advantage' is the definition.
(to queue-jump is to push in front in a queue)
'signal heard to unexpectedly attack' is the wordplay.
'signal' becomes 'cue' (as in an actor's cue).
'heard' indicates a 'sounds like' (homophone) clue.
'to unexpectedly attack' becomes 'jump' (to jump someone is to attack them suddenly).
'cue' sounds like 'queue'.
'queue'+'jump'='QUEUE-JUMP'
'and' acts as a link.
(Other definitions for queue-jump that I've seen before include "Go ahead of one's turn" , "Push in ahead of one's turn" , "Don't wait in line" , "Illegally push ahead of others" .)