Falls into ruin (6)
I believe the answer is:
decays
'falls into ruin' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for decays that I've seen before include "Rots and decomposes" , "Goes off" , "Rots, moulders away" , "Rots, disintegrates" , "Decomposes or falls into ruin" .)