Copper has energy for signal (3)
I believe the answer is:
cue
'signal' is the definition.
(as in an actor's cue)
'copper has energy' is the wordplay.
'copper' becomes 'cu' (Cu is the chemical symbol for copper).
'has' means one lot of letters go next to another.
'energy' becomes 'e' (physics symbol as in E=mcu).
'cu'+'e'='CUE'
'for' is the link.
(Other definitions for cue that I've seen before include "Hint, prompt" , "On a car[D?] this would give a prompt" , "strike" , "Potter" , "Catchword" .)