Plural "is" (3)
I believe the answer is:
are
'plural is' is the definition.
(plural form of the verb 'is')
This is the entire clue.
(Other definitions for are that I've seen before include "(They/you/we) exist" , "Unit of land measure equal to 100 square metres" , "Is, in the plural" , "Exist; square measure" , "is for more than one" .)