Shoot of tree beginning to emerge (4)
I believe the answer is:
Here is my best explanation:
'shoot' is the definition.
(firing is a kind of shooting)
'tree beginning to emerge' is the subsidiary indication.
'tree' becomes 'fir' (fir is a kind of tree).
'beginning to' indicates taking the first letters.
The first letter of 'emerge' is 'e'.
'of' is the link.
(Other definitions for fire that I've seen before include "Trigger", "See 5", "Flame - dismiss - shoot", "cut", "Ready, aim . . . .", "Discharge (gun)", "There's no smoke without it", "Blaze - discharge", "Set light to; shoot" and "Sack; discharge (a gun)".)