One standing in line with old copper and male (5)
I believe the answer is:
locum
'one standing in' is the definition.
(I've seen this before)
'line with old copper and male' is the wordplay.
'line with' becomes 'l' (used when specifying particular lines from a poem. I am not sure about the 'with' bit.).
'old' becomes 'o' (abbreviation).
'copper' becomes 'Cu' (chemical symbol).
'and' says to put letters next to each other.
'male' becomes 'm' (common abbreviation).
'l'+'o'+'cu'+'m'='LOCUM'
Can you help me to learn more?
(Other definitions for locum that I've seen before include "A substitute or stand-in doctor" , "One standing in" , "may step into someone else's job" , "One stands in" , "Professional who substitutes temporarily for another" .)