Firm keeping oxygen in store (5)
I believe the answer is:
hoard
'store' is the definition.
(I know that hoard is a type of store)
'firm keeping oxygen' is the wordplay.
'firm' becomes 'hard' (both can mean durable or strong).
'keeping' is an insertion indicator.
'oxygen' becomes 'O' (atomic symbol for oxygen).
'hard' placed around 'o' is 'HOARD'.
'in' is the link.
(Other definitions for hoard that I've seen before include "Secret store of valuables" , "Squirrel away" , "(Miser's) store" , "Amass and store privately" , "Accumulation" .)