When a yes conceals a no (3)
I believe the answer is:
nay
'a no' is the definition.
(thesaurus)
'when a yes conceals' is the wordplay.
'conceals' indicates the answer is hidden within the clue.
'NAY' can be found hidden inside 'when a yes'.
(Other definitions for nay that I've seen before include "No - a negative vote" , "Indeed" , "No (dialect); or rather" , "Archaic negative" , "Negative response" .)