Soapy buildup (4)
I believe the answer is:
scum
'soapy buildup' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for scum that I've seen before include "Impurities floating on water, or despicable people" , "Impurities that rise to surface of liquid" , "People regarded as worthless, especially criminals" , "Floating dirt or froth" , "Dirt on liquid surface" .)