The flat in Sevenoaks (4)
I believe the answer is:
even
'the flat' is the definition.
(level)
'in sevenoaks' is the wordplay.
'in' says the answer is hidden in the clue.
'EVEN' can be found hidden inside 'sevenoaks'.
(Other definitions for even that I've seen before include "Level, calm" , "Like 2, 4 or 6, say" , "50-50" , "Equable" , "Flat, level" .)