Unit of area (4)
'unit of area' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for acre that I've seen before include "stretch of land" , "A state of W Brazil" , "Four roods - Israeli port" , "Race (anag.)" , "Care about the land measure" .)
(Other definitions for hectare that I've seen before include "10,000 squaer metres" , "Surface area equal to 10,000 square meters" , "Metric measure of area" , "Metric unit of area" , "Extent" .)