A line is spoken as a prompt (3)
I believe the answer is:
cue
'a prompt' is the definition.
(cue can mean a prompt or signal)
'a line is spoken' is the wordplay.
'a line' becomes 'queue' (queue is a kind of line).
'is spoken' indicates a 'sounds like' (homophone) clue.
'queue' sounds like 'CUE'.
'as' acts as a link.
(Other definitions for cue that I've seen before include "Signal to begin - tapered stick" , "Catchword" , "Actor's prompt" , "Hint, prompt" , "line in speech" .)