Swallow that flies off when gun is fired (4)
I believe the answer is:
slug
I believe this is a double definition.
'swallow' is the first definition.
(slug can informally mean to drink)
'that flies off when gun is fired' is the second definition.
(slug can mean a bullet)
(Other definitions for slug that I've seen before include "Slimy fellow in bed" , "gulp!" , "ammunition?" , "Slimy gastropod" , "Small creature like snail without shell" .)