Copper infiltrates unit when required (2,3)
I believe the answer is:
on cue
'when required' is the definition.
(on cue means at the required time)
'copper infiltrates unit' is the wordplay.
'copper' becomes 'cu' (chemical symbol).
'infiltrates' indicates putting letters inside.
'unit' becomes 'one' (synonyms).
'cu' going within 'one' is 'ON CUE'.
(Other definitions for on cue that I've seen before include "At the correct moment, like efficient actor" , "Where a tip is appearing" , "Explanation as to how something might work" , "At the correct moment, like actor prompted to speak" , "When expected" .)