Least extensive (8)
I believe the answer is:
shortest
'least extensive' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for shortest that I've seen before include "She trots (anag.)" , "Least in length" , "most direct" , "'The longest way round is the ... way home' (proverb)" , "Most vertically challenged" .)