Copper with most unusual habit (6)
I believe the answer is:
custom
'habit' is the definition.
(thesaurus)
'copper with most unusual' is the wordplay.
'copper' becomes 'cu' (chemical symbol).
'with' says to put letters next to each other.
'unusual' indicates an anagram.
'most' is an anagram of 'stom'.
'cu'+'stom'='CUSTOM'
(Other definitions for custom that I've seen before include "Usual way of doing something" , "'Habit, practice (6)'" , "Usual practice" , "Traditional usage" , "Tradition; habit" .)