Copper most upset at practice (6)
I believe the answer is:
custom
'practice' is the definition.
(custom is a kind of practice)
'copper most upset' is the wordplay.
'copper' becomes 'cu' (chemical symbol).
'upset' is an anagram indicator.
'most' with letters rearranged gives 'stom'.
'cu'+'stom'='CUSTOM'
'at' acts as a link.
(Other definitions for custom that I've seen before include "Established usage" , "Accepted or habitual practice" , "Way of doing things" , "Habitual use" , "Traditional usage" .)