Mould blocking drain on the surface (4-4)

I believe the answer is:

skin-deep

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'on the surface' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

(Other definitions for skin-deep that I've seen before include "Superficial like body graze" , "Develop" , "Stringed instrument" , "Superficial (like beauty?)" , "Superficial as beauty is said to be" .)

I've seen this clue in The Guardian.

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