Extend the length of a sentence? (7)
I believe the answer is:
stretch
This clue is a triple definition!
'extend' is the first definition.
(I know that stretch is a kind of extend)
'the length' is the second definition.
(thesaurus)
'of' is a link.
'a sentence?' is the third definition.
(a stretch in prison)
(Other definitions for stretch that I've seen before include "Period inside" , "Strain" , "Bit of exercise" , "Period of time in prison" , "that one may do to a point" .)