The measure of an Israeli seaport (4)
I believe the answer is:
acre
This clue is a double definition.
'the measure' is the first definition.
(acre is a kind of measure)
'an israeli seaport' is the second definition.
(I've seen this in another clue)
'of' acts as a link.
(Other definitions for acre that I've seen before include "Persistent tendency" , "Care about the land measure" , "holy city" , "Old unit of area - old port" , "Israeli seaport" .)