Prepare to fire first (5)
I believe the answer is:
prime
I believe this is a triple definition!
'prepare' is the first definition.
(to prime is to prepare or make ready)
'to fire' is the second definition.
(thesaurus)
'first' is the third definition.
(similar in meaning)
(Other definitions for prime that I've seen before include "Number with no factor but itself and 1" , "Two or three, for example" , "2017, for example" , "Fundamental - of highest quality" , "Number such as two, three, five, seven, etc" .)