Mass of bubbles (4)
I believe the answer is:
foam
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">mass of 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 foam that I've seen before include "Fizz, froth" , "At the mouth, this may mean madness or extreme anger" , "Suds" , "Bubbles on liquid" , "Suspension of gas in a liquid" .)