Brunette seen with Arab -- mysterious candidate (4,5)
I believe the answer is:
dark horse
'mysterious candidate' is the definition.
(I know that dark horse is a type of mysterious candidate)
'brunette seen with arab' is the wordplay.
'brunette' becomes 'dark' (similar in meaning).
'seen with' is a charade indicator (letters next to each other).
'arab' becomes 'horse' (I've seen this in another clue).
'dark'+'horse'='DARK HORSE'
(Other definitions for dark horse that I've seen before include "unexpectedly successful entry" , "Candidate, not well known" , "Brown heroin" , "Unfancied winner" , "Black Beauty, for instance" .)