Mass of small bubbles (5)
I believe the answer is:
froth
'mass of small bubbles' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for froth that I've seen before include "Small bubbles on a liquid" , "Effervesce" , "Lather" , "Spume or foam" , "Mass of small bubbles" .)