Divisible by 2 (4)
I believe the answer is:
even
'divisible by 2' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for even that I've seen before include "When darkness starts to fall" , "Level, calm" , "Flush" , "Like 2,4,6, ..." , "Level, straight or regular" .)