A flat turned out to be lethal (5)
I believe the answer is:
fatal
'to be lethal' is the definition.
('fatal' can be similar in meaning to 'lethal')
'a flat turned' is the wordplay.
'turned' is an anagram indicator.
'a'+'flat'='aflat'
'aflat' with letters rearranged gives 'FATAL'.
'out' is the 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" .)