Ruin one metal flower (8)
I believe the answer is:
marigold
'flower' is the definition.
(I know that marigold is a type of flower)
'ruin one metal' is the wordplay.
'ruin' becomes 'mar' (I've seen this in another clue).
'one' becomes 'I' (Roman numeral).
'metal' becomes 'gold' (gold is a kind of metal).
'mar'+'i'+'gold'='MARIGOLD'
(Other definitions for marigold that I've seen before include "Orange daisy" , "The flower has a gold rim, oddly" , "Orange-flowered plant" , "Garden plant of the daisy family" , "Composite plant" .)