Charlie enters certain city in South America (5)
I believe the answer is:
sucre
'city in south america' is the definition.
(constitutional capital of Bolivia)
'charlie enters certain' is the wordplay.
'charlie' becomes 'C' (phonetic alphabet).
'enters' indicates putting letters inside.
'certain' becomes 'sure' (synonyms).
'c' inserted inside 'sure' is 'SUCRE'.
(Other definitions for sucre that I've seen before include "South American currency" , "sweetener abroad" , "City of Bolivia" , "Ecuadorean currency; sugar (French)" , "Bolivian city" .)