A move to split large South African city (5)
I believe the answer is:
lagos
'african city' is the definition.
(Nigerian city)
'a move to split large south' is the wordplay.
'a move' becomes 'ago' (I can't justify this - if you can you should believe this answer much more).
'to split' is an insertion indicator.
'large' becomes 'L' (eg in clothes sizes).
'south' becomes 'S' (synonyms).
'l'+'s'='ls'
'ago' put within 'ls' is 'LAGOS'.
(Other definitions for lagos that I've seen before include "Strange goals of the Nigerian port" , "Nigerian city and port" , "Former Nigerian capital" , "Formerly capital of Nigeria" , "city in Africa" .)