A sound white bucket (4)
I believe the answer is:
pail
'bucket' is the definition.
('pail' can be a synonym of 'bucket')
'a sound white' is the wordplay.
'a sound' shows a homophone (sound like).
'white' becomes 'pale' (I've seen this before).
'pale' is a homophone of 'PAIL'.
(Other definitions for pail that I've seen before include "Container; sounds like dim" , "Cylindrical vessel" , "Milkmaid's bucket?" , "it may go down well" , "water-carrier" .)