Disastrous scratch — patio ruined (12)
I believe the answer is:
catastrophic
'disastrous' is the definition.
(I've seen this before)
'scratch patio ruined' is the wordplay.
'ruined' indicates an anagram.
'scratch'+'patio'='scratchpatio'
'scratchpatio' with letters rearranged gives 'CATASTROPHIC'.
(Other definitions for catastrophic that I've seen before include "Extremely unfortunate" , "Chaotic parts (anag)" , "Causing sudden damage" , "Disastrous" .)