One sultan moves to annex British port (8)
I believe the answer is:
istanbul
'port' is the definition.
(I've seen this before)
'one sultan moves to annex british' is the wordplay.
'one' becomes 'i' (Roman numeral).
'moves' is an anagram indicator.
'to annex' means one lot of letters goes inside another (inserted letters get taken over or annexed).
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'sultan' is an anagram of 'stanul'.
'stanul' enclosing 'b' is 'stanbul'.
'i'+'stanbul'='ISTANBUL'
(Other definitions for istanbul that I've seen before include "Modern name of Constantinople" , "Catch the Latin bus out to Turkey" , "foreign port" , "It used to be named Constantinople" , "Eurasian city" .)