Clothes to wear when you're out? (8)

I believe the answer is:

nighties

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But I can't explain it at all! Could you help me understand?

(Other definitions for nighties that I've seen before include "habits adopted in retirement" , "Ladies' sleeping attire" , "Female clothing" , "Womens' wear" , "gear for bed" .)

I've seen this clue in The Guardian.

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