One can be a bit untidy (4)

I believe the answer is:

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'one' is the definition.
(synonyms)

I can't explain the remainder of the clue.

(Other definitions for unit that I've seen before include "Body of soldiers" , "Individual thing; one" , "Single item, maybe part of a set" , "Single element; one" , "A single, complete item" .)

I've seen this clue in the Evening Standard.

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