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