I am about to prepare to fire (3)
I believe the answer is:
aim
'prepare to fire' is the definition.
(I've seen this in another clue)
'i am about' is the wordplay.
'about' means one lot of letters goes inside another.
'i' put within 'am' is 'AIM'.
'to' acts as a link.
(Other definitions for aim that I've seen before include "Train" , "Point or direct, a weapon say" , "Plan" , "Set sights on, target or goal" , "Try to achieve" .)