Somewhat run-down ruin (4)
I believe the answer is:
undo
'ruin' is the definition.
(someone's undoing is their ruin)
'somewhat run-down' is the wordplay.
'somewhat' says the answer is hidden in the clue.
'UNDO' is hidden within 'run down'.
(Other definitions for undo that I've seen before include "Free from ties, laces, etc" , "Unfasten - ruin" , "Take back" , "Unravel or ruin" , "Ruin prospects of; cancel" .)