Mostly regretting bankruptcy (4)
I believe the answer is:
ruin
'bankruptcy' is the definition.
(thesaurus)
'mostly regretting' is the wordplay.
'mostly' means to remove the last letter.
'regretting' becomes 'ruing' ('rue' can be a synonym of 'regret').
'ruing' with its last letter taken away is 'RUIN'.
(Other definitions for ruin that I've seen before include "Utter destruction or downfall" , "State of devastation and destruction" , "Destroy or bankrupt" , "Physical collapse" , "dishonour" .)