A study of port (4)
I believe the answer is:
Here is my best explanation:
'port' is the definition.
'a study' is the subsidiary indication.
'study' becomes 'den' (I've seen this before).
'of' is the link.
(Other definitions for aden that I've seen before include "Capital of the Yemen", "Seaport in Yemen", "Port city of Yemen", "Main port of Yemen", "somewhere on the Arabian peninsula", "Dean out in the seaport city", "Dean is out in the Middle Eastern port", "Dane in seaport of Yemen", "Commercial capital of Yemen" and "Yemen's chief port".)