About to load the gun? Terrific! (5)
I believe the answer is:
great
'terrific' is the definition.
(I've seen this in another clue)
'about to load the gun?' is the wordplay.
'about' is an insertion indicator.
'to load' becomes 're' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'the gun?' becomes 'gat' (gat is a kind of gun).
're' placed inside 'gat' is 'GREAT'.
Can you help me to learn more?
(Other definitions for great that I've seen before include "Brunel's third and last steamship" , "Terrific" , "Of major significance - very good" , "cracking" , "Very large in size, quantity, etc." .)