Metric unit of area (3)
I believe the answer is:
are
'metric unit of area' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for are that I've seen before include "Exist; square measure" , "(We/they/you) exist" , "(We/you/they) exist" , "live" , "(They/you/we) exist" .)