Rejection of agreement covering national insurance (6)
I believe the answer is:
denial
'rejection' is the definition.
(rejection of a claim)
'agreement covering national insurance' is the wordplay.
'agreement' becomes 'deal' (deal is a kind of agreement).
'covering' indicates putting letters inside.
'national insurance' becomes 'NI' (UK tax).
'deal' placed around 'ni' is 'DENIAL'.
'of' acts as a link.
(Other definitions for denial that I've seen before include "Refusal to acknowledge" , "Negation" , "Rejection of a request" , "Refusal to admit painful truths" , "Nailed (anag.)" .)