Ruing endless destruction (4)
I believe the answer is:
ruin
'destruction' is the definition.
(ruin is a kind of destruction)
'ruing endless' is the wordplay.
'endless' means to remove the last letter.
'ruing' with its final letter taken away is 'RUIN'.
(Other definitions for ruin that I've seen before include "Destroy or bankrupt" , "loss of position" , "Spoil, impoverish" , "Wrecked building" , "financial disaster" .)