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", "Strange Dane to be in the Yemen", "Dane in seaport of Yemen", "Dean is out in the Middle Eastern port", "Yemen's chief port", "Dean out in the seaport city", "Yemeni port", "Yemeni gulf", "Port city of Yemen" and "Main port of Yemen".)