Copper that takes the biscuit showing restraint? (5)
I believe the answer is:
cuffs
'restraint?' is the definition.
(cuffs is a kind of restraint)
'copper that takes the biscuit' is the wordplay.
I cannot really see how this works, but
'copper' could be 'cu' (Cu is the chemical symbol for copper) and 'cu' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'showing' acts as a link.
Can you help me to learn more?
(Other definitions for cuffs that I've seen before include "Hits" , "Clips around the ear" , "Ends of sleeves" , "Sleeve folds" , "Open-handed blows" .)