Understand speaker's refusal (4)
I believe the answer is:
know
'understand' is the definition.
(thesaurus)
'speaker's refusal' is the wordplay.
'speaker's' shows a homophone (sound like).
'refusal' becomes 'no'.
'no' sounds like 'KNOW'.
(Other definitions for know that I've seen before include "Recognize" , "Ken" , "Have awareness of" , "A Channel port, type of sole" , "Be sure of something" .)