There's nothing in the department store (5)
I believe the answer is:
'store' is the definition.
(depot can mean a store or warehouse)
'there's nothing in the department' is the wordplay.
'there's nothing' becomes 'o' (I've seen this in other clues).
'in the' means one lot of letters goes inside another.
'department' becomes 'dept'.
'o' going within 'dept' is 'DEPOT'.
(Other definitions for depot that I've seen before include "Regimental HQ" , "bus terminus" , "Depository for goods" , "Headquarters - storehouse" , "Storage base" .)