Understand sound of the waves? (3)
I believe the answer is:
see
'understand' is the definition.
(I know that understand can be written as see)
'sound of the waves?' is the wordplay.
'sound of' shows a homophone (sound like).
'the waves?' becomes 'sea' (I've seen this in other clues).
'sea' is a homophone of 'SEE'.
(Other definitions for see that I've seen before include "Witness; ensure" , "Understand; view" , "date" , "around cathedral" , "Glimpse; get" .)