A giant so upset in South American capital (8)
I believe the answer is:
santiago
'south american capital' is the definition.
(capital of Chile)
'a giant so upset' is the wordplay.
'upset' indicates an anagram.
'a'+'giant'+'so'='agiantso'
'agiantso' anagrammed gives 'SANTIAGO'.
'in' acts as a link.
(Other definitions for santiago that I've seen before include "City" , "Capital of Chile" , "Chilean capital" , "Chile's capital" , "South American capital" .)