100 square metres (3)
I believe the answer is:
are
'100 square metres' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for are that I've seen before include "Era (anag.)" , "(We/they/you) exist" , "(They/you/we) exist" , "Exist; square measure" , "is for more than one" .)