What triggers prompter's hint when line's forgotten (3)
I believe the answer is:
cue
'what triggers prompter's' is the definition.
(as in a stage cue)
I don't understand how the rest of the clue works.
(Other definitions for cue that I've seen before include "Signal to players" , "Hint on stage" , "Actor's prompt" , "Tapering stick used by snooker player" , "Used in snooker and on stage" .)