Bankrupt regretted keeping fashionable (6)
I believe the answer is:
ruined
'bankrupt' is the definition.
(thesaurus)
'regretted keeping fashionable' is the wordplay.
'regretted' becomes 'rued' ('rue' can be a synonym of 'regret').
'keeping' means one lot of letters goes inside another.
'fashionable' becomes 'in' ('in' can be similar in meaning to 'fashionable').
'rued' enclosing 'in' is 'RUINED'.
(Other definitions for ruined that I've seen before include "Bankrup - destroyed" , "Bust" , "Reduced to poverty; damaged" , "Spoilt" , "Physically destroyed or financially impoverished" .)