Fifty spoil day for dame (4)
I believe the answer is:
lady
'for dame' is the definition.
(synonyms)
'fifty spoil day' is the wordplay.
'fifty' becomes 'l' (Roman numeral).
'spoil' is an anagram indicator.
'day' anagrammed gives 'ady'.
'l'+'ady'='LADY'
(Other definitions for lady that I've seen before include "Luck, for example" , "See 20" , "older female" , "Female title" , "See 19" .)