Exercise period in prison (7)
I believe the answer is:
stretch
I believe this is a double definition.
'exercise' is the first definition.
(stretch is a kind of exercise)
'period in prison' is the second definition.
(I've seen this before)
(Other definitions for stretch that I've seen before include "that will make a bigger man of you?" , "Make to go further" , "that one may do to a point" , "Spell of time (in prison?)" , "extended" .)