It's deadly re-doing a flat (5)
I believe the answer is:
fatal
'it's deadly' is the definition.
(similar in meaning)
'doing a flat' is the wordplay.
'doing' indicates an anagram (doing can mean solving or working out).
'a'+'flat'='aflat'
'aflat' anagrammed gives 'FATAL'.
're' is the link.
This may not be right. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for fatal that I've seen before include "Death-dealing" , "Mortal" , "Unavoidable" , "Disastrous, maybe lethal" , "Pernicious" .)