A flat turned out deadly (5)
I believe the answer is:
fatal
'deadly' is the definition.
('fatal' can be similar in meaning to 'deadly')
'a flat turned out' is the wordplay.
'turned out' is an anagram indicator.
'a'+'flat'='aflat'
'aflat' anagrammed gives 'FATAL'.
(Other definitions for fatal that I've seen before include "Involving death" , "Causing death, say accident" , "Pernicious" , "Bringing ruin" , "Calamitous" .)