Tiny chest, bust, is fake (9)
I believe the answer is:
synthetic
'is fake' is the definition.
(I've seen this before)
'tiny chest bust' is the wordplay.
'bust' is an anagram indicator.
'tiny'+'chest'='tinychest'
'tinychest' with letters rearranged gives 'SYNTHETIC'.
(Other definitions for synthetic that I've seen before include "Artificially produced" , "Not natural in origin" , "Mock" , "ersatz" , "Not genuine, unnatural" .)