Black Beauty? (4,5)
I believe the answer is:
dark horse
'black beauty?' is the definition.
(Black Beauty is a fictional black horse)
'black beauty?' is the wordplay.
'black' becomes 'dark' (synonyms).
'beauty?' becomes 'horse' (I can't explain this - if you can you should believe this answer much more).
'dark'+'horse'='DARK HORSE'
Can you help me to learn more?
(Other definitions for dark horse that I've seen before include "Candidate, not well known" , "Competitor about whose abilities little is known" , "Little-known competitor like shadowy equine" , "it may be foolish to discount" , "Successful competitor of whom little was known, like colourless equine" .)