Rejection of agreement about Northern Ireland (6)
I believe the answer is:
denial
'rejection' is the definition.
(I've seen this before)
'agreement about northern ireland' is the wordplay.
'agreement' becomes 'deal' (deal is a kind of agreement).
'about' indicates putting letters inside.
'northern ireland' becomes 'ni'.
'deal' going around 'ni' is 'DENIAL'.
'of' is the link.
(Other definitions for denial that I've seen before include "Negation" , "Repudiation" , "Renunciation" , "Refusal, abnegation" , "Nailed (anag.)" .)