Only admitting 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)
'only admitting copper' is the wordplay.
'only' becomes 'one' ('one' can be a synonym of 'only').
'admitting' indicates putting letters inside.
'copper' becomes 'cu' (chemical symbol).
'one' going around 'cu' is 'ON CUE'.
(Other definitions for on cue that I've seen before include "When expected" , "at the perfect moment" , "Explanation as to how something might work" , "you can find chalk here" , "When signalled" .)