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