The most common and most likely answer to this crossword clue is the 4 plus 5 letter word DARK HORSE
I've seen this crossword clue in The Times
D
A
R
K
H
O
R
S
E
More clues leading to the result DARK HORSE
- But a grey also may be so unfancied a runner
- Black Beauty's unexpected success?
- Unknown order has assembled around king
- Obscure thing used by a gymnast, an unknown quantity?
- Outsider provides rescue vessel, when surrounded by rampaging hordes
- Unknown quantity of diamonds and heroin around vessel
- Sinister drug, an unknown quantity
- Mysterious figure rode shark at sea
- Black Bess, for example, reveals unexpected talent
- Black Beauty, an unexpected winner?
Buddy explains!
The straight part
We know that "unexpected success" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unexpected success = dark horse.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.