South American most smart and thin (7)
I believe the answer is:
chilean
'south american' is the definition.
(Chilean is an example)
'most smart and thin' is the wordplay.
'most' means to remove the last letter.
'smart' becomes 'chic' (synonyms).
'and' means one lot of letters go next to another.
'thin' becomes 'lean' (synonyms).
'chic' with its last letter taken off is 'chi'.
'chi'+'lean'='CHILEAN'
(Other definitions for chilean that I've seen before include "from South American country" , "From e.g. Valparaiso" , "Nationality of citizen of Santiago" , "From Santiago, say?" , "Spanish speaker" .)