Flutter to be in (3)
I believe the answer is:
bat
I believe this is a double definition.
'flutter' is the first definition.
('bat' can be a synonym of 'flutter')
'be in' is the second definition.
(I've seen this before)
'to' acts as a link.
(Other definitions for bat that I've seen before include "Nocturnal mouselike mammal" , "A mammal; a club" , "Flutter (eyelids)" , "Small winged mammal, may be vampire" , "Small flying mammal, may be vampire" .)