Extend period of time in prison (7)
I believe the answer is:
stretch
I believe this is a triple definition!
'extend' is the first definition.
(I know that extend is a is a more specific form of the action of stretch)
'period' is the second definition.
(stretch of time)
'of' is a link.
'time in prison' is the third definition.
(I've seen this before)
(Other definitions for stretch that I've seen before include "Draw out" , "time spent in jail" , "Period inside" , "Period of time in prison" , "Make demands on; lengthen" .)