Signal made by copper on top of eminence (3)
I believe the answer is:
cue
'signal' is the definition.
(thesaurus)
'by copper on top of eminence' is the wordplay.
'by' says to put letters next to each other.
'copper' becomes 'cu' (chemical symbol).
'on top of' says to take the initial letters (I've seen 'on top' mean this).
The first letter of 'eminence' is 'e'.
'cu' next to 'e' is 'CUE'.
'made' acts as a link.
(Other definitions for cue that I've seen before include "A snooker ball striker" , "Billiard stick - hint - signal" , "Actor's signal" , "Actor's line to remind another" , "Snooker rod" .)