Be aware of hearing refusal (4)
I believe the answer is:
know
'be aware of' is the definition.
(I've seen this before)
'hearing refusal' is the wordplay.
'hearing' shows a homophone (sound like).
'refusal' becomes 'no'.
'no' is a homophone of 'KNOW'.
(Other definitions for know that I've seen before include "Recognise" , "Be acquainted with" , "Ken" , "Realise" , "Articulated" .)