Green’s Dictionary of Slang

a (bookmaker’s) clerk.

1898		Binstead & Wells Pink ’Un and a Pelican 229: The ‘After-Dark’ stooped to pick it up.
1902		Sun. Times (Perth) 1 June 4/6: One day his own particular clerk, ‘after dark,’ as he familiarly calls him, fell sick.
1903		Sun. Times (Perth) 9 Aug. 4/8: They hunted round for ‘After Darks’.
2013		(con. 1970s) T. Peacock More You Bet 14: Bill and I were Steve’s ‘bookmaker’s clerks’ or in rhyming slang, ’after darks’.