South Africa’s composer turning up in Croatian city (6)
I believe the answer is:
zagreb
'croatian city' is the definition.
(I've seen this in another clue)
'south africa's composer turning up' is the wordplay.
'south africa' becomes 'za' (abbreviation for Zuid-Afrika).
'composer' becomes 'berg' (20th Century Austrian composer).
'turning up' says the letters should be written in reverse.
'berg' reversed gives 'greb'.
'za'+'greb'='ZAGREB'
'in' is the link.
(Other definitions for zagreb that I've seen before include "Croatian city" , "European capital" , "on River Sava" , "Croatia's capital city" , "Croatian capital" .)