Not that odd number in Sevenoaks! (4)
I believe the answer is:
Here is my best explanation:
'not that odd number' is the definition.
I can't judge whether this defines the answer.
'in sevenoaks' is the wordplay.
'in' says the answer is hidden in the clue.
'EVEN' is hidden in the letters of 'sevenoaks'.
(Other definitions for even that I've seen before include "tied", "Smooth, level", "Calm", "Flat - yet", "Equally balanced; level", "Flat, regular", "It's not odd", "Regular; calm", "Level or smooth" and "Placid - not easily angered".)