Copper most upset by tradition (6)
I believe the answer is:
custom
'tradition' is the definition.
(I know that tradition can be written as custom)
'copper most upset' is the wordplay.
'copper' becomes 'Cu' (Cu is the chemical symbol for copper).
'upset' indicates anagramming the letters.
'most' anagrammed gives 'stom'.
'cu'+'stom'='CUSTOM'
'by' acts as a link.
(Other definitions for custom that I've seen before include "Local habit" , "Habit, usage" , "Way of doing things" , "Regular trade" , "Convention" .)