Crossword Genius

One's brothers' keeper? (6)

Ross

I believe the answer is:

friary

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This clue might be a double definition.

'one's' is the first definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

'brothers' keeper?' is the second definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

Can you help me to learn more?

(Other definitions for friary that I've seen before include "Religious establishment" , "One's home to order" , "Place of the mendicant orders" , "Religious institution" , "House of Dominicans, for example" .)

I've seen this clue in The Sun.
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