Heartless fool, say, making bet (7)
I believe the answer is:
flutter
'bet' is the definition.
(to have a flutter is to put a bet on)
'heartless fool say' is the wordplay.
'heartless' means to remove the middle letters.
'say' becomes 'utter' (to utter something is to say or speak it).
'fool' with its middle removed is 'fl'.
'fl'+'utter'='FLUTTER'
'making' is the link.
(Other definitions for flutter that I've seen before include "Small bet; beat feebly" , "Fly unsteadily; small bet" , "Informally a slight gamble as on a horse" , "Bat" , "Move one's eyelashes - move like a butterfly - bet" .)