Period of time in prison (7)
I believe the answer is:
stretch
'period of time in prison' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for stretch that I've seen before include "Extend, elongate" , "that one may do to a point" , "Period inside" , "Lengthen - time in jail" , "Period of time in prison" .)