Copper gets point of signal (3)
I believe the answer is:
'signal' is the definition.
(as in an actor's cue)
'copper gets point' is the wordplay.
'copper' becomes 'Cu' (chemical symbol).
'gets' means one lot of letters go next to another.
'point' becomes 'e' (compass point).
'of' is the link.
(Other definitions for cue that I've seen before include "Prompt; signal" , "Rod used in billiards" , "On a car[D?] this would give a prompt" , "Actor's prompt" , "Actor's words" .)