Endlessly regretting downfall (4)
I believe the answer is:
ruin
'downfall' is the definition.
(thesaurus)
'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 removed is 'RUIN'.
(Other definitions for ruin that I've seen before include "Utter destruction or downfall" , "Destory or bankrupt" , "Mar, lay waste to" , "Devastate" , "Destroy completely" .)