No deal including Northern Ireland (6)
I believe the answer is:
denial
'no' is the definition.
(I've seen this in another clue)
'deal including northern ireland' is the wordplay.
'including' means one lot of letters goes inside another.
'northern ireland' becomes 'NI' (abbreviation).
'deal' placed around 'ni' is 'DENIAL'.
(Other definitions for denial that I've seen before include "Refusal to admit painful truths" , "Daniel (anag.)" , "Repudiation" , "Rejection of a request" , "Declaration that something is untrue" .)