Substitute left over copper money (5)

I believe the answer is:

locum

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'substitute' is the definition.
(locum is a kind of substitute)

'left over copper money' is the wordplay.
'left over' becomes 'lo' (this could be a standard abbreviation of which I'm unaware).
'copper' becomes 'cu' (chemical symbol).
'money' becomes 'm' (abbreviation in economics).
'lo'+'cu'+'m'='LOCUM'

Can you help me to learn more?

(Other definitions for locum that I've seen before include "Medical temp?" , "He fills in" , "Relief doctor" , "Professional temp" , "temporary occupant" .)

I've seen this clue in The Independent.

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