Celebrities with a brand new South American flag (5,3,4)
I believe the answer is:
stars and bars
'american flag' is the definition.
(name for the flag of the USA)
'celebrities with a brand new south' is the wordplay.
'celebrities' becomes 'stars' (I've seen this in other clues).
'with' is a charade indicator (letters next to each other).
'new' is an anagram indicator.
'south' becomes 'S' (synonyms).
'brand' anagrammed gives 'ndbar'.
'stars'+'a'+'ndbar'+'s'='STARS AND BARS'
(Other definitions for stars and bars that I've seen before include "Flag of the Confederate States of America" , "once standard in America" .)