Warehouse salesman, old one in fiction (10)
I believe the answer is:
repository
'warehouse' is the definition.
(I've seen this before)
'salesman old one in fiction' is the wordplay.
'salesman' becomes 'rep' (I've seen this before).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'fiction' becomes 'story' (story is a kind of fiction**).
'i' inserted within 'story' is 'sitory'.
'rep'+'o'+'sitory'='REPOSITORY'
(Other definitions for repository that I've seen before include "Warehouse" , "cabinet" , "Box containing items for safekeeping" , "dump?" , "Storage place" .)