Is, pluralized (3)
I believe the answer is:
are
'is pluralized' 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 "Unit of land measure equal to 100 square metres" , "Era (anag.)" , "Exist; square measure" , "is for more than one" , "(They/you/we) exist" .)