It's a rigid rule (9)

I believe the answer is:

yardstick

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'rule' is the definition.
(I know that Yardstick is a type of rule)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for yardstick that I've seen before include "Guideline" , "Benchmark" , "measure of performance" , "Measuring rod" , "Measure against which to make comparison" .)

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

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