Substitute delivered when copper put in large order (5)

I believe the answer is:

locum

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'substitute' is the definition.
(locum can mean a substitute)

'copper put in large order' is the wordplay.
'copper' becomes 'Cu' (chemical symbol).
'put in' is an insertion indicator.
'large' becomes 'L' (eg in clothes sizes).
'order' becomes 'om' (Order of Merit).
'l'+'om'='lom'
'cu' inserted into 'lom' is 'LOCUM'.

'delivered when' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.

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(Other definitions for locum that I've seen before include "Relief doctor" , "Someone standing in temporarily for another of the same profession" , "Temporary deputy for doctor" , "Stand-in physician" , "Doctor's stand-in" .)

I've seen this clue in The Independent.

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