Pond film (4)
I believe the answer is:
scum
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">pond film</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for scum that I've seen before include "Floating film of filth" , "Impurities on top of water or worthless people" , "Impure film" , "Matter floating on 9 ac" , "Film on the surface of a liquid" .)