Not even half a mile outside Sevenoaks! (4)
I believe the answer is:
less
'not' is the definition.
(I've seen this in other clues)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for less that I've seen before include "Wanting" , "Minus; not so much" , "Smaller in amout or degree" , "Comparative of 'a little'" , "A smaller amount" .)