Mass of small bubbles (5)
I believe the answer is:
froth
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">mass of small bubbles</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for froth that I've seen before include "Small bubbles on a liquid" , "Effervesce" , "Lather" , "Spume or foam" , "Mass of small bubbles" .)