Firm keeping nothing in store (5)
'store' is the definition.
(hoard is a kind of store)
'firm keeping nothing' is the wordplay.
'firm' becomes 'hard' (both can mean durable or strong).
'keeping' is an insertion indicator.
'nothing' becomes 'o' (looks like zero - 0).
'hard' going around 'o' is 'HOARD'.
'in' is the link.
(Other definitions for hoard that I've seen before include "One's savings", "Squirrel away", "Collection", "accumulated wealth", "Hidden stock".)