Prepare to fire mate (4)
I believe the answer is:
cock
I believe this is a double definition.
'prepare to fire' is the first definition.
(as in cocking a gun)
'mate' is the second definition.
(I've seen this before)
(Other definitions for cock that I've seen before include "early bird" , "Male lobster; valve" , "Tilt (one's head to one's side)" , "Make gun ready" , "Adult male bird" .)