Farmland measurement (4)
I believe the answer is:
acre
'farmland measurement' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for acre that I've seen before include "Unit of area - Israeli port" , "Levant sea port" , "once occupied by Crusaders" , "Old unit of area - old port" , "Unit of land area" .)