Is plural (3)
I believe the answer is:
are
'is plural' is the definition.
(plural form of the verb 'is')
This is all the clue.
(Other definitions for are that I've seen before include "(We/you/they) exist" , "is for more than one" , "(They/you/we) exist" , "Is, in the plural" , "Era (anag.)" .)