Unusually little done (4)

I believe the answer is:

rare

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'done' is the definition.
Both the definition and answer are adjectives. Perhaps they are linked in a way I don't understand?

I can't explain the rest of the clue.

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(Other definitions for rare that I've seen before include "Seldom seen or met" , "having few matches?" , "Excellent; uncommon" , "Exceptional" , "not dense" .)

I've seen this clue in the Evening Standard.

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