Hazardous thing to do, you can be sure! (3)

I believe the answer is:

bet

I'm a little stuck... Click here to teach me more about this clue!

'be sure' is the definition.
('I bet that...' means 'I am sure that...')

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for bet that I've seen before include "Speculation" , "Wager, or the sum waged" , "Money risked on a gamble" , "'Stake, wager (3)'" , "Wager, back" .)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app