Soapy water (4)
I believe the answer is:
suds
'soapy water' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for suds that I've seen before include "Lather" , "Soapy water" , "Foam from detergent" , "Soapy froth" , "Mass of soap bubbles" .)