Increase prison sentence (7)
I believe the answer is:
stretch
I believe this is a double definition.
'increase' is the first definition.
(stretch is a kind of increase)
'prison sentence' is the second definition.
(Other definitions for stretch that I've seen before include "that one may do to a point" , "Period of time in prison" , "Exaggerate" , "Draw out" , "Physical exercise" .)