Exercise done by a prisoner (7)
I believe the answer is:
stretch
'exercise' is the definition.
(stretching is a kind of exercising)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(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" .)