Prolong the sentence (7)
I believe the answer is:
stretch
This clue is a double definition.
'prolong' is the first definition.
(thesaurus)
'sentence' is the second definition.
(a stretch in prison)
'the' acts as a link.
(Other definitions for stretch that I've seen before include "Continuous expanse/period" , "Draw out" , "Make to go further" , "Pull to make longer or wider" , "Prison sentence (colloquial)" .)