Rebuttal rejected in breaking agreement (6)
I believe the answer is:
denial
'rebuttal' is the definition.
(thesaurus)
'rejected in breaking agreement' is the wordplay.
'rejected' says the letters should be written backwards.
'breaking' means one lot of letters goes inside another.
'agreement' becomes 'deal' (deal is a kind of agreement**).
'in' back-to-front is 'ni'.
'ni' placed within 'deal' is 'DENIAL'.
(Other definitions for denial that I've seen before include "Negation" , "Nailed (anag.)" , "Refusal to admit painful truths" , "Daniel (anag.)" , "Refusal, abnegation" .)