Long shot (4,5)
I believe the answer is:
dark horse
'long shot' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for dark horse that I've seen before include "Little is known of its properties" , "One about whom little is known" , "unexpectedly successful entry" , "it may be foolish to discount" , "Unfancied winner" .)