Non-existence (7)
I believe the answer is:
absence
'non-existence' is the definition.
(I know that non-existence can be written as absence)
This is the entire clue.
(Other definitions for absence that I've seen before include "Non-presence" , "It makes the heart grow fonder, they say" , "Need" , "Seen cab (anag.)" , "Failure to show" .)