Surprise candidate (4,5)
I believe the answer is:
dark horse
'surprise candidate' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for dark horse that I've seen before include "it may be foolish to discount" , "Competitor of unknown capabilities" , "Unknown quantity" , "One about whom little is known" , "Unexpected (possible) winner" .)