Part of the gun the shopkeeper holds (5)
I believe the answer is:
stock
'part of the gun' is the definition.
('stock' is part of 'gun')
'the shopkeeper holds' is the wordplay.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'shopkeeper' becomes 'sock' (I can't justify this - if you can you should believe this answer much more).
'holds' indicates putting letters inside.
't' going into 'sock' is 'STOCK'.
Can you help me to learn more?
(Other definitions for stock that I've seen before include "shop's saleable items" , "Broth" , "Supply; part of rifle" , "F < for traditional" , "raw material" .)