Ray's bottom is starting to get dirty (5)

I believe the answer is:

smear

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'dirty' is the definition.
(smearing is a kind of dirtying)

I don't understand the remainder of the clue.

(Other definitions for smear that I've seen before include "Greasy or sticky mark" , "Blot or daub" , "allegation" , "material for examination that is on the slide" , "Spread a greasy or sticky substance over something" .)

I've seen this clue in The Independent.

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