Rejection of deal about Northern Ireland (6)
I believe the answer is:
denial
'rejection' is the definition.
(rejection of a claim)
'deal about northern ireland' is the wordplay.
'about' means one lot of letters goes inside another.
'northern ireland' becomes 'NI' (abbreviation).
'deal' placed around 'ni' is 'DENIAL'.
'of' acts as a link.
(Other definitions for denial that I've seen before include "Refusal to admit painful truths" , "Saying it isn't so" , "Renunciation" , "Refusal to accept an unpalatable truth" , "Refusal to acknowledge" .)