Bet there will be a flap (7)
I believe the answer is:
flutter
I believe this is a double definition.
'bet' is the first definition.
(I have seen 'Small bet' mean 'flutter' so perhaps 'bet' could also mean 'flutter')
'a flap' is the second definition.
('flutter' can be a synonym of 'flap')
'there will be' is the link.
(Other definitions for flutter that I've seen before include "Informally a slight gamble as on a horse" , "Bat" , "Fly unsteadily; small bet" , "Move one's eyelashes - move like a butterfly - bet" , "Small bet; beat feebly" .)