Little known candidate's a nightmare? (4,5)
I believe the answer is:
dark horse
'little known candidate's' is the definition.
'dark horse' can be an answer for 'candidate's' (dark horse is a kind of candidate). I am not sure about the 'little known' bit.
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for dark horse that I've seen before include "Unfancied winner" , "unexpectedly successful entry" , "Black Beauty, for instance" , "Successful competitor of whom little was known, like colourless equine" , "Unexpected (possible) winner" .)