People in line overheard signal for action (3)
I believe the answer is:
cue
'signal for action' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for cue that I've seen before include "Actor's line to remind another" , "strike" , "Hint, prompt" , "On a car[D?] this would give a prompt" , "Sign" .)