Usual practice of copper’s cat (6)
I believe the answer is:
custom
'usual practice' is the definition.
(I've seen this before)
'copper's cat' is the wordplay.
'copper's' becomes 'cu's' (chemical symbol).
'cat' becomes 'tom' (a tom is a male cat).
'cus'+'tom'='CUSTOM'
'of' acts as a link.
(Other definitions for custom that I've seen before include "Tradition; habit" , "Usual way of doing something" , "Traditional usage" , "Local habit" , "Regular process" .)