Repudiate participants in Greek overreaction after upheaval (6)
I believe the answer is:
revoke
'repudiate' is the definition.
(thesaurus)
'participants in greek overreaction after upheaval' is the wordplay.
'participants in' indicates the answer is hidden within the clue.
'after upheaval' says the letters should be written backwards (in a down clue, letters go up).
'ekover' is hidden in the letters of 'greek overreaction'.
'ekover' back-to-front is 'REVOKE'.
(Other definitions for revoke that I've seen before include "Annul, rescind" , "Withdraw or cancel a decree" , "Annul (a decree/decision)" , "Withdraw or cancel, say a licence" , "Cancel or rescind, say a licence" .)