Air defence space in port (4)
I believe the answer is:
aden
'port' is the definition.
(port in Yemen)
'air defence space' is the wordplay.
'air defence' becomes 'AD' (military abbreviation).
'space' becomes 'en' (in printing, a space as wide as a letter 'n').
'ad'+'en'='ADEN'
'in' acts as a link.
(Other definitions for aden that I've seen before include "Dean out in the seaport city" , "Yemeni port" , "foreign climes" , "Dean is in the Yemen, strangely" , "somewhere on the Arabian peninsula" .)