Rotten cold one caught in South African capital (6)
I believe the answer is:
rancid
'rotten' is the definition.
(rancid can mean rotten or foul)
'cold one caught in south african capital' is the wordplay.
'cold' becomes 'C' (eg on taps).
'one' becomes 'i' (Roman numeral).
'caught in' means one lot of letters goes inside another.
'south african capital' becomes 'rand' (currency of South Africa).
'c'+'i'='ci'
'ci' placed within 'rand' is 'RANCID'.
(Other definitions for rancid that I've seen before include "Rank, high" , "Sour like stale grease 96)" , "Rank in smell and taste" , "Foul-tasting" , "Smelly and going bad" .)