Instrument of torture that's associated with ruin (4)
I believe the answer is:
rack
I believe this clue is a double definition.
'instrument of torture' is the first definition.
(rack is a kind of instrument of torture)
'associated with ruin' is the second definition.
(I've seen this before)
'that's' is the link.
(Other definitions for rack that I've seen before include "Shelf for luggage" , "Rail for torture" , "Storage frame; draw off (wine)" , "Cause of anguish" , "Luggage carrier - instrument of torture" .)