Person brings in copper at the right moment (2,3)
I believe the answer is:
on cue
'at the right moment' is the definition.
(I've seen this before)
'person brings in copper' is the wordplay.
'person' becomes 'one' (I've seen this in other clues).
'brings in' is an insertion indicator.
'copper' becomes 'cu' (chemical symbol).
'one' going around 'cu' is 'ON CUE'.
(Other definitions for on cue that I've seen before include "At the correct moment, like actor prompted to speak" , "Explanation as to how something might work" , "promptly?" , "Right on time" , "As a snooker player might be" .)