Bet it's a flat-bottomed boat! (4)
I believe the answer is:
punt
I believe this is a double definition.
'bet' is the first definition.
(I know that punt is a more specific form of the action bet)
'it's a flat-bottomed boat' is the second definition.
(I've seen this in another clue)
(Other definitions for punt that I've seen before include "Pre-euro currency unit of Ireland" , "Flat boat for Irish money" , "risky affair" , "Gamble -- boat -- high kick" , "taken by a better" .)