Gamble on flying start before turning turtle (7)
I believe the answer is:
flutter
'gamble' is the definition.
(I've seen this before)
'flying start before turning turtle' is the wordplay.
'start' says to take the initial letters.
'before' says to put letters next to each other.
'turning' indicates an anagram.
The initial letter of 'flying' is 'f'.
'turtle' with letters rearranged gives 'lutter'.
'f'+'lutter'='FLUTTER'
'on' is the link.
(Other definitions for flutter that I've seen before include "Move one's eyelashes - move like a butterfly - bet" , "Bat" , "Small bet; beat feebly" , "Fly unsteadily; small bet" , "Informally a slight gamble as on a horse" .)