Plaintiff upset about copper's hesitation (7)
I believe the answer is:
accuser
'plaintiff' is the definition.
(thesaurus)
'upset about copper's hesitation' is the wordplay.
'upset' says the letters should be written in reverse.
'about' becomes 'ca' (abbreviation for 'circa'**).
'copper's' becomes 'cus' (chemical symbol).
'hesitation' becomes 'er'.
'ca' backwards is 'ac'.
'ac'+'cus'+'er'='ACCUSER'
(Other definitions for accuser that I've seen before include "origin of charge?" , "who tries to blame someone else" , "I may charge" , "One bringing charges" , "One who blames" .)