Religious adherent sues firm, getting 50% (4)

I believe the answer is:

sufi

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'religious' is the definition.
Both the answer and definition are singular nouns.
Maybe you can see a link between them that I don't see?

I cannot understand how the rest of the clue works.

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(Other definitions for sufi that I've seen before include "Islamic ascetic" , "Muslim mystic" , "Moslem mystic" , "Muslim inclined to mysticism" .)

I've seen this clue in The Irish Times.

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