Stage prompt for line heard (3)
I believe the answer is:
cue
'stage prompt' is the definition.
(I've seen this in another clue)
'line heard' is the wordplay.
'line' becomes 'queue' (queue is a kind of line).
'heard' shows a homophone (sound like).
'queue' is a homophone of 'CUE'.
'for' is the link.
(Other definitions for cue that I've seen before include "Prompt; reminder" , "Tapering stick used by snooker player" , "Snooker rod" , "signal to come on" , "Billiards stick" .)