Stretching of legs authorised by precedent? (14)

I believe the answer is:

constitutional

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'authorised by precedent?' is the definition.
'constitutional' can be an answer for 'authorised' (thesaurus). I'm not certain of the 'by precedent?' bit.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for constitutional that I've seen before include "Governmental" , "consistent with our government" , "like our rulers" , "in line with country code" , "steps taken to improve health" .)

I've seen this clue in The Times.

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