"That's ___ fair!" (3)
(Other definitions for not that I've seen before include "Negatory word" , "Negation of what follows" , "A negative word" , "Word of negation" , "Ton (anag.)" .)
'that's fair' is the definition.
(thesaurus)
This is all the clue.
(Other definitions for valid that I've seen before include "Well-grounded" , "Effectual" , "Legally acceptable and sound" , "Officially acceptable" , "Based on reason or law" .)