Real thin with a cute bust! (9)
I believe the answer is:
authentic
'real' is the definition.
(thesaurus)
'thin with a cute bust' is the wordplay.
'with' means one lot of letters go next to another.
'bust' is an anagram indicator.
'a'+'cute'='acute'
'thin' after 'acute' is 'acutethin'.
'acutethin' with letters rearranged gives 'AUTHENTIC'.
(Other definitions for authentic that I've seen before include "trustworthy" , "Bona fide" , "Genuine, not a copy" , "24 [LEGIT]" , "reliable" .)