Difficult bottling oxygen for store (5)
I believe the answer is:
hoard
'store' is the definition.
(hoard is a kind of store)
'difficult bottling oxygen' is the wordplay.
'difficult' becomes 'hard' ('hard' can be a synonym of 'difficult').
'bottling' is an insertion indicator (bottling is a kind of containing).
'oxygen' becomes 'o' (atomic symbol for oxygen).
'hard' enclosing 'o' is 'HOARD'.
'for' acts as a link.
(Other definitions for hoard that I've seen before include "Stash of treasure" , "Store, usually hidden" , "keep in reserve" , "Squirrel away" , "accumulated wealth" .)