The most common and most likely answer to this crossword clue is the 6 letter word BANKER
I've seen this crossword clue in The Daily Telegraph
B
A
N
K
E
R
More clues leading to the result BANKER
Buddy explains!

The straight part
We know that "a sure bet" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a sure bet = banker.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.
What does BANKER mean?
BANKER noun- One who conducts the business of banking; one who, individually, or as a member of a company, keeps an establishment for the deposit or loan of money, or for traffic in money, bills of exchange, etc.
- A money changer.
- The dealer, or one who keeps the bank in a gambling house.
- The stone bench on which a mason cuts or squares his work.
- A vessel employed in the cod fishery on the banks of Newfoundland.
- A ditcher; a drain digger.
- A banksman.
- A railway locomotive that can be attached to the rear of a train to assist it in climbing an incline.