Be up (3)
I believe the answer is:
bat
'be up' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for bat that I've seen before include "Wooden implement for cricket" , "Nocturnal mouselike mammal" , "Flutter (eyelids)" , "Small mammal or cricket striker" , "Small flying mammal, may be vampire" .)