"But the fellow has left," you say (7)
I believe the answer is:
flutter
'but the' is the definition.
I can't tell whether this defines the answer.
'fellow has left you say' is the wordplay.
'fellow' becomes 'f' (abbreviation - of a society etc.).
'has' is a charade indicator (letters next to each other).
'left you' becomes 'l' (I am not sure about this - if you are sure you should believe this answer much more).
'say' becomes 'utter' (to utter something is to say or speak it).
'f'+'l'+'utter'='FLUTTER'
Can you help me to learn more?
(Other definitions for flutter that I've seen before include "Bat" , "Move one's eyelashes - move like a butterfly - bet" , "Small bet; beat feebly" , "Fly unsteadily; small bet" , "Informally a slight gamble as on a horse" .)