Not this or that (7)
I believe the answer is:
neither
'not this or that' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for neither that I've seen before include "Not one thing or the other" , "In three (anag.)" , "Therein lies not one or the other" , "Therein (anag)" , "both are rejected" .)