A study in port (4)
I believe the answer is:
aden
'port' is the definition.
(I've seen this before)
'a study' is the wordplay.
'study' becomes 'den' (I've seen this before).
'a'+'den'='ADEN'
'in' acts as a link.
(Other definitions for aden that I've seen before include "Yemen's chief port" , "somewhere on the Arabian peninsula" , "Strange Dane to be in the Yemen" , "Commercial capital of Yemen" , "foreign climes" .)