Shoot of tree beginning to emerge (4)
I believe the answer is:
'shoot' is the definition.
(firing is a kind of shooting)
'tree beginning to emerge' is the wordplay.
'tree' becomes 'fir' (fir is a kind of tree).
'beginning to' suggests taking the first letters.
The initial letter of 'emerge' is 'e'.
'of' acts as a link.
(Other definitions for fire that I've seen before include "Sack; discharge (a gun)" , "Sack; conflagration" , "Ready, aim . . . ." , "... or combustion" , "Set light to; shoot" .)