Copper with most unusual habit (6)
I believe the answer is:
Here is my best explanation:
'habit' is the definition.
(I've seen this before)
'copper with most unusual' is the subsidiary indication.
'copper' becomes 'cu' ('Cu' can be a synonym of 'copper').
'with' is a charade indicator (letters next to each other).
'unusual' indicates an anagram.
'most' anagrammed gives 'stom'.
(Other definitions for custom that I've seen before include "Habitual practice", "Usual way of doing something", "Habitual patronage", "Convention", "Traditional habit", "Traditional activity or practice", "Traditional usage", "Local habit", "Established usage" and "Tradition; habit".)