A flat turned out to be lethal (5)
I believe the answer is:
fatal
'to be lethal' is the definition.
(similar in meaning)
'a flat turned' is the wordplay.
'turned' indicates anagramming the letters.
'a'+'flat'='aflat'
'aflat' anagrammed gives 'FATAL'.
'out' acts as a link.
(Other definitions for fatal that I've seen before include "Disastrous, maybe lethal" , "Resulting in disaster or death" , "Causing or leading up to death" , "Involving death" , "Calamitous" .)