A flat, in disorder, is deadly (5)
I believe the answer is:
fatal
'is deadly' is the definition.
(similar in meaning)
'a flat in disorder' is the wordplay.
'in disorder' indicates anagramming the letters.
'a'+'flat'='aflat'
'aflat' anagrammed gives 'FATAL'.
(Other definitions for fatal that I've seen before include "very bad for health!" , "Unavoidable" , "Disastrous, maybe lethal" , "Bringing death" , "Mortal" .)