Deal in strange contradiction (6)
I believe the answer is:
denial
'contradiction' is the definition.
(I've seen this in another clue)
'deal in strange' is the wordplay.
'strange' indicates anagramming the letters.
'deal'+'in'='dealin'
'dealin' is an anagram of 'DENIAL'.
(Other definitions for denial that I've seen before include "Negation" , "Refusal to acknowledge" , "Refusal to believe an unacceptable truth" , "Declaration that something is untrue" , "Act of asserting that something alleged is not true" .)