Operation performed in coat (6)
I believe the answer is:
action
'operation' is the definition.
(thesaurus)
'performed in coat' is the wordplay.
'performed' indicates an anagram.
'in'+'coat'='incoat'
'incoat' is an anagram of 'ACTION'.
(Other definitions for action that I've seen before include "Performance" , "A judicial proceeding brought by one party against another" , "Battle (in the law courts?)" , "Film director's cry?" , "Lawsuit; armed conflict" .)