Imitating a noise of a bullet (5)
I believe the answer is:
aping
'imitating' is the definition.
('ape' can be a synonym of 'imitate')
'a noise of a bullet' is the wordplay.
'noise of a bullet' becomes 'ping' (I can't justify this - if you can you should believe this answer much more).
'a'+'ping'='APING'
Can you help me to learn more?
(Other definitions for aping that I've seen before include "Sending up" , "Acting like a parrot" , "Imitating, copying" , "Trying to repeat" , "Taking off" .)