Too long unmarried, but presumably immediately available? (2,3,5)
I believe the answer is:
on the shelf
This clue is a double definition.
'too long unmarried' is the first definition.
(on the shelf can mean single or unmarried)
'presumably immediately available?' is the second definition.
(available in store)
'but' is the link.
(Other definitions for on the shelf that I've seen before include "destined never to meet her beau?" , "Where you might find Emma" , "Book here, perhaps" , "Despairing of a husband" .)