A flat became deadly (5)
I believe the answer is:
fatal
'deadly' is the definition.
(similar in meaning)
'a flat became' is the wordplay.
'became' indicates anagramming the letters (I've seen 'become' mean this).
'a'+'flat'='aflat'
'aflat' is an anagram of 'FATAL'.
(Other definitions for fatal that I've seen before include "Leading to disaster" , "Bringing ruin" , "Causing or leading up to death" , "Unavoidable" , "Calamitous" .)