Not one of those outgoing types? (4-2-4)

I believe the answer is:

stay-at-home

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'not one' is the definition.
Both the answer and definition are adjectives. Maybe there's a link between them I don't understand?

I cannot understand how the rest of the clue works.

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(Other definitions for stay-at-home that I've seen before include "One not given to travel" , "Person disinclined to travel" , "Unadventurous in travel" , "Don't go out" .)

I've seen this clue in the Evening Standard.

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