It's tough with nothing in store (5)
I believe the answer is:
hoard
'store' is the definition.
(hoard is a kind of store)
'it's tough with nothing' is the wordplay.
'it's tough' becomes 'hard' ('hard' can be a synonym of 'tough').
'with' is an insertion indicator.
'nothing' becomes 'o' (looks like zero - 0).
'hard' placed around 'o' is 'HOARD'.
'in' is the link.
(Other definitions for hoard that I've seen before include "Accumulated store" , "Secret store of valuables" , "(Miser's) store" , "A lot of people" , "Valuable store" .)