Real thin with a cute bust! (9)
I believe the answer is:
'real' is the definition.
'thin with a cute bust' is the wordplay.
'with' means one lot of letters go next to another.
'bust' is an anagram indicator.
'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" .)