Coppers must accept an imposed punishment (7)
I believe the answer is:
penance
'imposed punishment' is the definition.
'coppers must accept an' is the wordplay.
'coppers' becomes 'pence' (I've seen this before).
'must accept' means one lot of letters goes inside another.
'pence' enclosing 'an' is 'PENANCE'.
(Other definitions for penance that I've seen before include "act of expiation" , "Voluntary self-punishment" , "Expression of sorrow for sin" , "Punishment for wrongdoing, by religion perhaps" , "Act performed in sorrow for sin" .)