Developing a flat may be deadly (5)
I believe the answer is:
fatal
'be deadly' is the definition.
(similar in meaning)
'developing a flat' is the wordplay.
'developing' indicates anagramming the letters (letters developed into a new form).
'a'+'flat'='aflat'
'aflat' with letters rearranged gives 'FATAL'.
'may' acts as a link.
(Other definitions for fatal that I've seen before include "Leading to disaster" , "Disastrous, maybe lethal" , "very bad for health!" , "critical" , "Deadly force" .)