Duke leaves South African port for city (5)
I believe the answer is:
urban
'city' is the definition.
(urban means relating to a city)
'duke leaves south african port' is the wordplay.
'duke' becomes 'd' (abbreviation).
'leaves' suggests deleting specific letters.
'south african port' becomes 'Durban' (port city in South Africa).
'durban' with 'd' taken away is 'URBAN'.
'for' acts as a link.
(Other definitions for urban that I've seen before include "Citified" , "City - pope" , "Town (adj.)" , "Had this description of town" , "Relating to towns" .)