Spoil one gilt flower (8)
I believe the answer is:
marigold
'flower' is the definition.
(marigold is a kind of flower)
'spoil one gilt' is the wordplay.
'spoil' becomes 'mar' (synonyms).
'one' becomes 'I' (Roman numeral).
'gilt' becomes 'gold' (synonyms).
'mar'+'i'+'gold'='MARIGOLD'
(Other definitions for marigold that I've seen before include "Composite plant" , "Orange daisy" , "floral item" , "Yellow or orange flower" , "Flower - grim load (anag)" .)