A flat becomes deadly (5)
I believe the answer is:
fatal
'deadly' is the definition.
('fatal' can be similar in meaning to 'deadly')
'a flat becomes' is the wordplay.
'becomes' indicates anagramming the letters.
'a'+'flat'='aflat'
'aflat' anagrammed gives 'FATAL'.
(Other definitions for fatal that I've seen before include "Pernicious" , "Bringing death" , "Disastrous, maybe lethal" , "making one late" , "Death-dealing" .)