Green’s Dictionary of Slang

a minor journalist.

1842		Whip & Satirist of NY & Brooklyn (NY) 8 Jan. n.p.: Thy wretched liners, and more wretched hacks.
1866		Sportsman 6 Sept. 2/1: Notes on News [...] The same love of verbal ‘larkiness’ that pervades soul of the ‘liner’ would seem to have entered into that of the ingenious writer of the first leader.
1870		Sportsman (London) 19 Jan. 2/1: The irrepressible ‘liner’—[is] not nowadays the penny-a-liner, as people unconnected with newspapers imagine, seeing that he turns up his nose at the mention of such a sum.
1882		Sydney Sl. Dict. (2 edn) 5: Liner - A casual reporter, paid by the line. Diminutive of ‘penny-a-liner’.
1895		C. Crowe Aus. Sl. Dict. 45: Liner, or Penny-a-Liner, a reporter paid by the line.
1895		Mirror of Life 18 May 10/3: [T]he sporting liner, the bookmaker, the sporting tradesman, or the amateur boxer.