Rejection of agreement restricting one part of UK (6)
I believe the answer is:
denial
'rejection' is the definition.
(I've seen this before)
'agreement restricting one part of uk' is the wordplay.
'agreement' becomes 'deal' (deal is a kind of agreement**).
'restricting' indicates putting letters inside.
'one part of uk' becomes 'ni' (Northern Ireland).
'deal' placed around 'ni' is 'DENIAL'.
'of' acts as a link.
(Other definitions for denial that I've seen before include "Act of asserting that something alleged is not true" , "Rejection or refusal" , "Declaration that something is untrue" , "Refusal, abnegation" , "Refusal to accept an unpalatable truth" .)