Modest bet (7)
I believe the answer is:
flutter
'modest bet' is the definition.
(flutter can mean a small bet)
This is all the clue.
(Other definitions for flutter that I've seen before include "Bat" , "Small bet; beat feebly" , "Fly unsteadily; small bet" , "Move one's eyelashes - move like a butterfly - bet" , "Informally a slight gamble as on a horse" .)