Extend period of imprisonment (7)
I believe the answer is:
stretch
This could be a double definition.
'extend' is the first definition.
(I know that extend can be written as stretch)
'period of imprisonment' is the second definition.
'stretch' can be an answer for 'period' (stretch of time). I'm not certain of the 'of imprisonment' bit.
Can you help me to learn more?
(Other definitions for stretch that I've seen before include "Pull in opposite directions" , "Extend, elongate" , "Period inside" , "how much time one's been given" , "Period of time in prison" .)