Wreck party after upsetting Greek character (4)
I believe the answer is:
undo
'wreck' is the definition.
(thesaurus)
'party after upsetting greek character' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'after' means one lot of letters go next to another.
'upsetting' says the letters should be written backwards.
'greek character' becomes 'nu' (letter from the Greek alphabet).
'nu' written backwards gives 'un'.
'do' put after 'un' is 'UNDO'.
(Other definitions for undo that I've seen before include "Loosen; cancel" , "set free" , "Release fastening" , "Remove the wrapping" , "Loosen; annul" .)