A flat turned out to be deadly (5)
I believe the answer is:
fatal
'to be deadly' is the definition.
(similar in meaning)
'a flat turned' is the wordplay.
'turned' indicates an anagram.
'a'+'flat'='aflat'
'aflat' is an anagram of 'FATAL'.
'out' acts as a link.
(Other definitions for fatal that I've seen before include "Death-dealing" , "Pernicious" , "Deadly force" , "Causing or leading up to death" , "critical" .)