As in sales clerk. Ring up sales, stock shelves, putter about and put things in order. Read a book or knock out some pushups if it's a really slow day, everything's in order, and the owner's not a dick. That kind of thing.

[EDIT] sense 5 in Webster's 1913. Apparently it is, or at least was, a US usage.

5. An assistant in a shop or store. [U. S.]