Copper’s cat’s habitual practice (6)
I believe the answer is:
custom
'habitual practice' is the definition.
(I've seen this before)
'copper's cat's' is the wordplay.
'copper's' becomes 'Cu's' (chemical symbol).
'cat' becomes 'tom' (a tom is a male cat).
'cus'+'tom'='CUSTOM'
(Other definitions for custom that I've seen before include "Traditional usage" , "Convention" , "'Habit, practice (6)'" , "Traditional activity or practice" , "Tradition; habit" .)