He will add barrels to his stocks (8)
I believe the answer is:
gunsmith
'his stocks' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he will add barrels' is the wordplay.
I cannot really understand how this works, but
'he' could be 'smith' and 'smith' is found in the answer.
'barrels' could be 'gun' ('gun barrel' can be a synonym of 'barrel') and 'gun' is present in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'to' acts as a link.
Can you help me to learn more?
(Other definitions for gunsmith that I've seen before include "Weapons maker" , "I can arm you" , "Arms maker" , "arms dealer" , "Maker of firearms" .)