Bathtub buildup (4)
I believe the answer is:
scum
'bathtub buildup' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for scum that I've seen before include "Dregs (of society?)" , "Worthless group of people" , "Impurities on surface of liquid, or rotten people" , "lowlife" , "Refuse" .)