Rigid forms of worship (4)

I believe the answer is:

pews

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'rigid forms' is the definition.
Although both the answer and definition are plural nouns, I can't understand how one could define the other.

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

Can you help me to learn more?

(Other definitions for pews that I've seen before include "Church seating" , "Church benches" , "Church seats" .)

I've seen this clue in The Guardian and The Globe and Mail.

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