Habit of copper’s cat (6)
I believe the answer is:
custom
'habit' is the definition.
(I've seen this before)
'copper's cat' is the wordplay.
'copper's' becomes 'Cu's' (Cu is the chemical symbol for copper).
'cat' becomes 'tom' (a tom is a male cat).
'cus'+'tom'='CUSTOM'
'of' is the link.
(Other definitions for custom that I've seen before include "Usual way of doing something" , "''Age cannot wither her, nor ... stale (Antony and Cleopatra)" , "'Habit, practice (6)'" , "Traditional usage" , "Regular process" .)