Criminal act of clergyman (not foremost of priests) (5)

I believe the answer is:

arson

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'criminal act' is the definition.
(arson is a crime)

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

(Other definitions for arson that I've seen before include "Fire-raiser's crime" , "Roans (anag)" , "Act of a pyromaniac" , "Iniquitous behaviour" , "Criminal burning of property" .)

I've seen this clue in The Telegraph.

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