Bit of earth on tree shoot (4)
I believe the answer is:
fire
'shoot' is the definition.
(firing is a kind of shooting)
'bit of earth on tree' is the wordplay.
'bit of' suggests taking the first letters.
'on' means one lot of letters go next to another.
'tree' becomes 'fir' (fir tree is a kind of tree).
The first letter of 'earth' is 'e'.
'e' after 'fir' is 'FIRE'.
(Other definitions for fire that I've seen before include "Set light to; shoot" , "Trigger" , "Dismiss; bake in a kiln" , "Ready, aim . . . ." , "Inspire" .)