Rejection of trade across Northern Ireland (6)
I believe the answer is:
denial
'rejection' is the definition.
(I've seen this before)
'trade across northern ireland' is the wordplay.
'trade' becomes 'deal' ('deal' can be a synonym of 'trade').
'across' is an insertion indicator.
'northern ireland' becomes 'ni'.
'deal' placed around 'ni' is 'DENIAL'.
'of' acts as a link.
(Other definitions for denial that I've seen before include "Renunciation" , "Act of asserting that something alleged is not true" , "Refusal to believe an unacceptable truth" , "Daniel (anag.)" , "Refusal to accept an unpalatable truth" .)