No deal about Northern Ireland! (6)
I believe the answer is:
denial
'no' is the definition.
(I've seen this before)
'deal about northern ireland' is the wordplay.
'about' indicates putting letters inside.
'northern ireland' becomes 'NI' (abbreviation).
'deal' enclosing 'ni' is 'DENIAL'.
(Other definitions for denial that I've seen before include "Renunciation" , "Act of asserting that something alleged is not true" , "Repudiation" , "Refusal to admit painful truths" , "Saying it isn't so" .)