Signal failure to put line in this, perhaps? (3)
I believe the answer is:
cue
'signal' is the definition.
(as in an actor's cue)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for cue that I've seen before include "Signal for player" , "Actor's signal" , "Hint on stage" , "Reminder to do something" , "Sports implement" .)