Lack; non-existence (7)
I believe the answer is:
absence
This clue is a double definition.
'lack' is the first definition.
(I know that absence is a type of lack)
'non-existence' is the second definition.
(I know that non-existence can be written as absence)
(Other definitions for absence that I've seen before include "Truancy" , "Need" , "Lack - it makes the heart grow fonder" , "Non-presence" , "Abstraction" .)