Copper gets point of signal (3)
I believe the answer is:
cue
'signal' is the definition.
(as in an actor's cue)
'copper gets point' is the wordplay.
'copper' becomes 'Cu' (chemical symbol).
'gets' says to put letters next to each other.
'point' becomes 'e' (compass point).
'cu'+'e'='CUE'
'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" .)