Fly-by-night! (3)
I believe the answer is:
bat
'fly-by-night' is the definition.
(animal which flies at night)
'fly-by' is the wordplay.
'fly' becomes 'b' (I can't explain this - if you can you should believe this answer much more).
'by' becomes 'at'.
'b'+'at'='BAT'
Can you help me to learn more?
(Other definitions for bat that I've seen before include "Cricket implement or flying mammal" , "Pipistrelle" , "Nocturnal flier" , "once wielded by 60 [GREIG]" , "Small winged mammal, may be vampire" .)