Tumbledown building (4)
I believe the answer is:
ruin
'tumbledown building' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ruin that I've seen before include "building now demolished" , "state of collapse" , "Mar, lay waste to" , "Spoil, impoverish" , "Disaster, collapse" .)