Realise negative answer, say (4)
I believe the answer is:
know
'realise' is the definition.
(I know that realise is a more specific form of know)
'negative answer say' is the wordplay.
'negative answer' becomes 'no'.
'say' shows a homophone (sound like).
'no' is a homophone of 'KNOW'.
(Other definitions for know that I've seen before include "Have information" , "Have awareness of" , "Articulated" , "Be aware of (something)" , "Realise" .)