Copper's method rejected imprisoning delinquent finally as formality (6)
I believe the answer is:
custom
'formality' is the definition.
(thesaurus)
'copper's method rejected' is the wordplay.
'copper's' becomes 'cus' (chemical symbol).
'method' becomes 'mot' (I can't explain this - if you can you should give a lot more credence to this answer).
'rejected' shows that the letters should be reversed in order.
'mot' back-to-front is 'tom'.
'cus'+'tom'='CUSTOM'
'imprisoning delinquent finally as' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for custom that I've seen before include "Convention" , "Habitual use" , "Traditional activity or practice" , "Usual way of doing something" , "Local habit" .)