Metric area unit
I believe the answer is:
are
'metric area unit' 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 "Unit of land measure equal to 100 square metres" , "Exist; square measure" , "live" , "(We/they/you) exist" , "is for more than one" .)