Gainsay the good (3)
I believe the answer is:
bad
'the good' is the definition.
(thesaurus)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for bad that I've seen before include "Stinking" , "Harmful" , "Rotten, evil" , "Evil, below standard or off" , "(Of food) decayed" .)