Flourished outside at one, causing ruin (5)
I believe the answer is:
fatal
'causing ruin' is the definition.
(I've seen this in another clue)
'flourished outside at one' is the wordplay.
'flourished' becomes 'fl' (I've seen this in other clues).
'outside' means one lot of letters goes inside another.
'one' becomes 'a'.
'at'+'a'='ata'
'fl' placed around 'ata' is 'FATAL'.
(Other definitions for fatal that I've seen before include "critical" , "Disastrous, maybe lethal" , "Deadly force" , "Calamitous" , "Causing or leading up to death" .)