Not a positive denial (8)
I believe the answer is:
negative
This clue is a triple definition!
'not' is the first definition.
(thesaurus)
'a positive' is the second definition.
(thesaurus)
'denial' is the third definition.
(I know that negative is a type of denial)
(Other definitions for negative that I've seen before include "Early form of photo" , "Less than positive" , "Piece of photographic film" , "Expressing or implying denial or refusal" , "veto" .)