Refusal to be crooked in deal (6)
I believe the answer is:
denial
'refusal' is the definition.
(thesaurus)
'crooked in deal' is the wordplay.
'crooked' is an anagram indicator.
'in'+'deal'='indeal'
'indeal' is an anagram of 'DENIAL'.
'to be' is the link.
(Other definitions for denial that I've seen before include "Nailed (anag.)" , "Declaration that something is untrue" , "Rejection or refusal" , "Saying it isn't so" , "Repudiation" .)