Dismissal of agreement about Ulster (6)
I believe the answer is:
denial
'dismissal' is the definition.
(thesaurus)
'agreement about ulster' is the wordplay.
'agreement' becomes 'deal' (deal is a kind of agreement).
'about' means one lot of letters goes inside another.
'ulster' becomes 'ni' (Northern Ireland).
'deal' enclosing 'ni' is 'DENIAL'.
'of' acts as a link.
(Other definitions for denial that I've seen before include "Refusal to admit painful truths" , "Declaration that something is untrue" , "Refusal to acknowledge" , "Saying it isn't so" , "Refusal to accept an unpalatable truth" .)