Not one or the other (7)
I believe the answer is:
neither
'not one or the other' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for neither that I've seen before include "And not; nor yet" , "Therein lies not one or the other" , "double negative?" , "The rein (anag.)" , "Not one of two" .)