Plural of is (3)
I believe the answer is:
are
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">plural of is</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)