Measure of port (4)
I believe the answer is:
acre
This clue is a double definition.
'measure' is the first definition.
(acre is a kind of measure)
'port' is the second definition.
(I've seen this before)
'of' is the link.
(Other definitions for acre that I've seen before include "Crusaders battlefield, now in Israel" , "Israeli seaport" , "A state of W Brazil" , "Measure of land area" , "point four nought five of a hectare" .)