Extend prison term (7)
I believe the answer is:
stretch
This clue is a double definition.
'extend' is the first definition.
(I know that extend can be written as stretch)
'prison term' is the second definition.
(I've seen this before)
(Other definitions for stretch that I've seen before include "Period inside" , "Elasticity, prison term" , "Make demands on; continuous expanse" , "Bit of exercise" , "sort of limousine" .)