Endlessly regretting penury (4)
I believe the answer is:
Here is my best explanation:
'penury' is the definition.
The answer and definition can be both states of affairs as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'endlessly regretting' is the wordplay.
'endlessly' means to remove the last letter.
'regretting' becomes 'ruing' ('rue' can be a synonym of 'regret').
'ruing' with its last letter taken off is 'RUIN'.
Can you help me to learn more?