An idle form of rejection (6)
I believe the answer is:
denial
'rejection' is the definition.
(rejection of a claim)
'an idle form' is the wordplay.
'form' is an anagram indicator (another form of the letters).
'an'+'idle'='anidle'
'anidle' anagrammed gives 'DENIAL'.
'of' acts as a link.
(Other definitions for denial that I've seen before include "Refusal to acknowledge" , "Renunciation" , "Rejection of a request" , "Negation" , "Nailed (anag.)" .)