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' (Alban Berg, 1885 - 1935).
'turning up' says the letters should be written backwards.
'berg' back-to-front is 'greb'.
'za'+'greb'='ZAGREB'
'in' acts as a link.
(Other definitions for zagreb that I've seen before include "Croatian city" , "European capital" , "on River Sava" , "Croatia's capital city" , "Croatian capital" .)