Plural of is (3)
I believe the answer is:
'plural of is' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for are that I've seen before include "(They/you/we) exist" , "Era (anag.)" , "Unit of land measure equal to 100 square metres" , "(We/they/you) exist" , "(We/you/they) exist" .)