Not singular (6)
I believe the answer is:
plural
'not singular' is the definition.
('plural' is the opposite to 'singular')
This is the entire clue.
(Other definitions for plural that I've seen before include "Denotes more than one" , "Consisting of more than one" , "More than one, not singular" , "Words such as "sheep" and "deer" (but not "cat" and "dog") can be this" , "Numbering more than one" .)