Endlessly regretting destruction (4)
I believe the answer is:
Here is my best explanation:
'destruction' is the definition.
(ruin is a kind of destruction)
'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 final letter taken off is 'RUIN'.