Not plural (8)
I believe the answer is:
singular
'not plural' is the definition.
('singular' is the opposite to 'plural')
This is the entire clue.
(Other definitions for singular that I've seen before include "Unusual - not plural" , "Odd; unique" , "Gnu's lair (anag.)" , "As sheep may be, but not cattle or horses" , "Unique - nails rug (anag)" .)