Usual copper's to spoil end of day (9)
I believe the answer is:
customary
'usual' is the definition.
(I've seen this before)
'copper's to spoil end of day' is the wordplay.
'copper's' becomes 'cus' (chemical symbol).
'spoil' becomes 'mar' (I've seen this before**).
'end of' indicates one should take the final letters.
The last letter of 'day' is 'y'.
'cus'+'to'+'mar'+'y'='CUSTOMARY'
(Other definitions for customary that I've seen before include "Commonly used or practised" , "Conventional" , "precedent" , "Courts may find this usual" , "Habitual, usual" .)