A flat in ruins? Deadly (5)
I believe the answer is:
fatal
'deadly' is the definition.
(similar in meaning)
'a flat in ruins?' is the wordplay.
'in ruins?' is an anagram indicator (I've seen 'in ruins' mean this).
'a'+'flat'='aflat'
'aflat' is an anagram of 'FATAL'.
(Other definitions for fatal that I've seen before include "Deadly force" , "Bringing death" , "critical" , "Causing or leading up to death" , "Causing death, say accident" .)