Firm with nothing in stock (5)
I believe the answer is:
hoard
'stock' is the definition.
(I know that hoard is a type of stock)
'firm with nothing' is the wordplay.
'firm' becomes 'hard' (both can mean durable or strong).
'with' indicates putting letters inside.
'nothing' becomes 'O' (O resembles 0).
'hard' going around 'o' is 'HOARD'.
'in' acts as a link.
(Other definitions for hoard that I've seen before include "Stash of treasure" , "Hard kick" , "Squirrel away" , "Amass secretly" , "Rhoda (anag.)" .)