A gunsmith adds it to his stock (6)
I believe the answer is:
barrel
This clue might be a double definition.
'a gunsmith adds it to his' is the first definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'stock' is the second definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
Can you help me to learn more?
(Other definitions for barrel that I've seen before include "Cylinder with metal hoops" , "Tube of a gun" , "Cask, drum" , "Round wooden cask made by cooper" , "Cheerful old song: 'Roll out the . . . . . .'" .)