Untidy, a flat can be deadly! (5)
I believe the answer is:
fatal
'be deadly' is the definition.
(similar in meaning)
'untidy a flat' is the wordplay.
'untidy' indicates anagramming the letters.
'a'+'flat'='aflat'
'aflat' is an anagram of 'FATAL'.
'can' is the link.
(Other definitions for fatal that I've seen before include "Unavoidable" , "Leading to disaster" , "Causing death, say accident" , "Bringing death" , "Resulting in disaster or death" .)