Positively denied involvement (6)
I believe the answer is:
indeed
'positively' is the definition.
(thesaurus)
'denied involvement' is the wordplay.
'involvement' indicates an anagram.
'denied' is an anagram of 'INDEED'.
(Other definitions for indeed that I've seen before include "Without question" , "Really - truly" , "Certainty" , "beyond doubt" , "Really - what a surprise!" .)