Dirty Uncle Andrew displays this (7)

I believe the answer is:

unclean

I'm a little stuck... Click here to teach me more about this clue!

'dirty' is the definition.
(I know that dirty can be written as unclean)

I don't understand the rest of the clue.

(Other definitions for unclean that I've seen before include "Filthy" , "Defiled" , "Spotted" , "Dirty, ritually impure" , "Infected" .)

I've seen this clue in the Sydney Morning Herald.

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