Shoot beginning to emerge beneath tree (4)
I believe the answer is:
'shoot' is the definition.
(firing is a kind of shooting)
'beginning to emerge beneath tree' is the wordplay.
'beginning to' suggests taking the first letters.
'beneath' says to put letters next to each other (in a down clue).
'tree' becomes 'fir' (fir tree is a kind of tree).
The initial letter of 'emerge' is 'e'.
'e' after 'fir' is 'FIRE'.
(Other definitions for fire that I've seen before include "cut" , "Flames; passion" , "There's no smoke without it" , "Inspire" , "Weaponry" .)