Spoil setter's yellow flower (8)
I believe the answer is:
marigold
'flower' is the definition.
(I know that marigold is a type of flower)
'spoil setter's yellow' is the wordplay.
'spoil' becomes 'mar' (synonyms).
'setter' becomes 'I' (the person who set this crossword).
'yellow' becomes 'gold' (gold is a kind of yellow).
'mar'+'i'+'gold'='MARIGOLD'
(Other definitions for marigold that I've seen before include "The flower has a gold rim, oddly" , "floral item" , "Bright orange-yellow garden flower" , "Garden plant of the daisy family" , "Composite plant" .)