A king entering South Africa after old Japanese city (5)
I believe the answer is:
osaka
'japanese city' is the definition.
(I've seen this before)
'a king entering south africa after old' is the wordplay.
'king' becomes 'k'.
'entering' means one lot of letters goes inside another.
'south africa' becomes 'sa' (abbreviation).
'after' is a charade indicator (letters next to each other).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'a'+'k'='ak'
'ak' placed into 'sa' is 'saka'.
'saka' put after 'o' is 'OSAKA'.
(Other definitions for osaka that I've seen before include "S. Japan port, bay" , "Kinki's capital" , "Major port and commercial and industrial centre on Honshu" , "Asian" , "Port city on Honshu, Japan" .)