Lengthen sentence (7)
I believe the answer is:
stretch
This clue is a double definition.
'lengthen' is the first definition.
(I know this)
'sentence' is the second definition.
(a stretch in prison)
(Other definitions for stretch that I've seen before include "Extend, time spent in prison" , "Period of time in prison" , "Extend; term in prison" , "Pull to make longer or wider" , "What one does in prison" .)