Saying it isn't so (6)
I believe the answer is:
denial
'saying it isn't so' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for denial that I've seen before include "Act of asserting that something alleged is not true" , "Repudiation" , "Refusal to accept an unpalatable truth" , "Refusal, abnegation" , "Renunciation" .)