Soapy stuff (4)
I believe the answer is:
suds
'soapy stuff' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for suds that I've seen before include "Lather" , "Mass of soap bubbles" , "Soapy froth" , "Froth made from soap" , "Lathery bubbles" .)