Habit makes copper most upset (6)
I believe the answer is:
custom
'habit' is the definition.
(thesaurus)
'copper most upset' is the wordplay.
'copper' becomes 'cu' (chemical symbol).
'upset' indicates an anagram.
'most' is an anagram of 'stom'.
'cu'+'stom'='CUSTOM'
'makes' is the link.
(Other definitions for custom that I've seen before include "Traditional usage" , "Usual practice" , "Habitual patronage" , "Accepted or habitual practice" , "Tradition; habit" .)