Extensive exercise done by prisoners (7)
I believe the answer is:
stretch
'exercise done by prisoners' is the definition.
'stretch' can be an answer for 'exercise' (stretching is a kind of exercising). I am not sure about the 'done by prisoners' bit.
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for stretch that I've seen before include "Bit of exercise" , "time at Pentonville?" , "Prison sentence (colloquial)" , "Extend, time spent in prison" , "sort of limousine" .)