Crossword Genius

A commotion’s beginning in church — natural to laugh (10)

Ross

I believe the answer is:

cachinnate

'laugh' is the definition.
(cachinnating is a kind of laughing)

I cannot understand how the rest of the clue works.

(Other definitions for cachinnate that I've seen before include "guffaw" , "laugh" .)

I've seen this clue in The Times.
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