Regret about writer, bankrupt, ultimately (6)
I believe the answer is:
repent
'regret' is the definition.
(synonyms)
'about writer bankrupt ultimately' is the wordplay.
'about' becomes 're' (common abbreviation for 'with reference to').
'writer' becomes 'pen' (I've seen this before).
'ultimately' indicates one should take the final letters.
The final letter of 'bankrupt' is 't'.
're'+'pen'+'t'='REPENT'
(Other definitions for repent that I've seen before include "Feel sorry" , "Show remorse" , "lying on the ground" , "Feel sincere regret" , "Feel or express contrition" .)