Monks or non-conformists in Plymouth? (8)

I believe the answer is:

brethren

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'monks' is the definition.
(I've seen this before)

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

(Other definitions for brethren that I've seen before include "Colleagues (in sect)" , "Fellow Christians or members of a religious order" , "Fellow males in an order" , "Fellow members of a sect" , "like John and Charles Wesley, perhaps" .)

I've seen this clue in The Sun.

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