Hearing of lines, actors may need these (4)
I believe the answer is:
cues
'actors may need these' is the definition.
'hearing of lines' is the wordplay.
'hearing of' shows a homophone (sound like).
'lines' becomes 'queues' (queue is a kind of line).
'queues' is a homophone of 'CUES'.
(Other definitions for cues that I've seen before include "Actor's signal" , "Snooker rods" , "Actors' signals" , "Signals for action" , "Prompts for action" .)