A flat may be deadly (5)
I believe the answer is:
fatal
'be deadly' is the definition.
(similar in meaning)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for fatal that I've seen before include "Disastrous, maybe lethal" , "Involving death" , "Pernicious" , "Bringing ruin" , "making one late" .)