Firm keeps duck in store (5)
I believe the answer is:
hoard
'store' is the definition.
(I know that hoard is a type of store)
'firm keeps duck' is the wordplay.
'firm' becomes 'hard' (both can mean durable or strong).
'keeps' is an insertion indicator.
'duck' becomes 'o' (resembles zero - 'duck' in cricket).
'hard' placed around 'o' is 'HOARD'.
'in' acts as a link.
(Other definitions for hoard that I've seen before include "Accumulation" , "Collection" , "(Miser's) store" , "One's savings" , "Amass secretly" .)