Crossword Genius

As worn by a Bobby at closing time? (4)

Ross

I believe the answer is:

robe

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'as' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for robe that I've seen before include "Clerical garment" , "Dressing-gown" , "Old cloak for Boer" , "official dress" , "habit" .)

I've seen this clue in the Evening Standard.
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