Club for a fly-by-night? (3)
I believe the answer is:
bat
I believe this clue is a double definition.
'club' is the first definition.
(as in sporting equipment)
'a fly-by-night?' is the second definition.
(animal which flies at night)
'for' is the link.
(Other definitions for bat that I've seen before include "Small flying mammal, may be vampire" , "Small mammal or cricket striker" , "Nocturnal flier" , "A mammal; a club" , "Night-flyer" .)