Firm keeping old stock (5)
I believe the answer is:
hoard
'stock' is the definition.
(hoard is a kind of stock)
'firm keeping old' is the wordplay.
'firm' becomes 'hard' ('hard' can be similar in meaning to 'firm').
'keeping' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'hard' going around 'o' is 'HOARD'.
(Other definitions for hoard that I've seen before include "Valuable store" , "Amass secretly" , "Stockpile" , "Secret store of valuables" , "Reserve supply" .)