Exercise done by a prisoner (7)

I believe the answer is:

stretch

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'exercise' is the definition.
(stretching is a kind of exercising)

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

(Other definitions for stretch that I've seen before include "that one may do to a point" , "Extend; term in prison" , "Time lag's got" , "how much time one's been given" , "extended" .)

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

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