Do limbering up exercises for a while (7)

I believe the answer is:

stretch

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'while' is the definition.
(I've seen this before)

I can't explain the rest of the clue.

(Other definitions for stretch that I've seen before include "Draw out" , "Strain" , "Continuous expanse/period" , "What one does in prison" , "Make (something) longer" .)

I've seen this clue in the Evening Standard.

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