Firm carrying old stock (5)
I believe the answer is:
hoard
'stock' is the definition.
(hoard is a kind of stock)
'firm carrying old' is the wordplay.
'firm' becomes 'hard' ('hard' can be similar in meaning to 'firm').
'carrying' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'hard' placed around 'o' is 'HOARD'.
(Other definitions for hoard that I've seen before include "Secret stash" , "Stash of treasure" , "Store of treasure" , "(Miser's) store" , "Store, usually hidden" .)