A mass of small bubbles (4)
I believe the answer is:
foam
'a mass of small bubbles' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for foam that I've seen before include "Bubbles on liquid" , "Sea froth" , "At the mouth, this may mean madness or extreme anger" , "Suds" , "Boiling bubbles" .)