Go away, only half firing (4)
I believe the answer is:
shoo
'go away' is the definition.
(I've seen this before)
'only half firing' is the wordplay.
'only half' suggests halving the letters.
'firing' becomes 'shooting' (shooting is a kind of firing).
'shooting' halved is 'SHOO'.
(Other definitions for shoo that I've seen before include "Get lost!" , "Sling your hook!" , "Begone!" , "word of dismissal" , "Exclamation used to chase away animals" .)