Where shopper may be arriving soon (2,5)
I believe the answer is:
in store
'arriving soon' is the definition.
(if something is 'in store' it's imminent)
I don't understand how the remainder of the clue works.
(Other definitions for in store that I've seen before include "stocked" , "Put by" , "Soon to happen" , "expected to appear" , "at the supermarket" .)