(n): a worker who is hired to perform a job
(n): a salesperson in a store
(n): a person employed to represent a business and to sell its merchandise (as to customers in a store or to customers who are visited)
(v): work as a clerk, as in the legal business
(v): exert oneself by doing mental or physical work for a purpose or out of necessity
v.i. work as a clerk – I've never come across this verbal use before. (OED marks it as colloquial now, but has examples back to 1551.)
I can remember all the tenants of the front room upstairs, who came and went: Vernie, who clerked in a store; the fabulous Doc Marlowe, who made and sold Sioux Liniment and wore a ten-gallon hat with kitchen matches stuck in the band; the blonde and mysterious Mrs Lane, of the strong perfume and the elegant dresses; Mr Richardson, a guard at the penitentiary, who kept a gun in his room; and a silent, thin, smiling man who never revealed his business and left with his rent two weeks in arrears.
—James Thurber, 1952, 'Daguerreotype of a Lady', in The Thurber Album