Fashionable thinner bust (9)
I believe the answer is:
insolvent
'bust' is the definition.
(I've seen this before)
'fashionable thinner' is the wordplay.
'fashionable' becomes 'in' ('in' can be similar in meaning to 'fashionable').
'thinner' becomes 'solvent' ('solvent' can be a synonym of 'thinner').
'in'+'solvent'='INSOLVENT'
(Other definitions for insolvent that I've seen before include "gone to the wall" , "without the means to pay" , "Broke" , "on the rocks" , "unable to raise the wind" .)