A measure of port (4)
I believe the answer is:
acre
I believe this is a double definition.
'a measure' is the first definition.
(acre is a kind of measure)
'port' is the second definition.
(I've seen this before)
'of' acts as a link.
(Other definitions for acre that I've seen before include "Imperial land measurement" , "once occupied by Crusaders" , "Old unit of area - old port" , "Unit of land area" , "Site of battle" .)