To fire (5)
I believe the answer is:
shoot
'to fire' is the definition.
(I know that shoot is a more specific form of the action discharge)
This is all the clue.
(Other definitions for shoot that I've seen before include "Fire (a gun)" , "Make a film" , "Kill; part of plant" , "New growth; last word of Hamlet" , "Film; move rapidly" .)