Individual bringing in copper when required (2,3)
I believe the answer is:
on cue
'when required' is the definition.
(on cue means at the required time)
'individual bringing in copper' is the wordplay.
'individual' becomes 'one' (an individual is one person).
'bringing in' is an insertion indicator.
'copper' becomes 'Cu' (chemical symbol).
'one' enclosing 'cu' is 'ON CUE'.
(Other definitions for on cue that I've seen before include "Explanation as to how something might work" , "When signalled" , "Where chalk may be put" , "As a snooker player might be" , "Right on time" .)