Make longer sentence (7)
I believe the answer is:
stretch
I believe this clue is a double definition.
'make longer' is the first definition.
(stretching is a kind of making longer)
'sentence' is the second definition.
(a stretch in prison)
(Other definitions for stretch that I've seen before include "time spent in jail" , "Pull to make longer or wider" , "Straighten the arms" , "that will make a bigger man of you?" , "Exercise - period" .)