Green’s Dictionary of Slang

1. (US) a handicapper, bookmaker or racing tipster who bases their information on timing the horses on their morning exercise.

1904		T.A. Dorgan in Zwilling TAD Lex. (1993) 27: All the ‘clockers’ were up early.
1906		Seattle Star (WA) 19 Oct. 2/1: The ‘clocker’ gets out before dawn and [...] clicks the time in which every extended horse breezes through the stretch.
1910		N.Y. Age 27 Jan. 6/6: Joe Yeager, the noted plunger, is said to have pulled off a a big betting coup [...] all brought about by the information Yeager received from his colored ‘clockers’.
1916		Honolulu Star-Bulletin (HI) 21 Feb. 17/3: He is one of the cleverest sporting editors in all the British Isles and has been the official ‘clocker’ for the [Daily] Telegraph for ever since I can remember.
1923		T.A. Dorgan in Zwilling TAD Lex. (1993) 94: I only bet what my clocker gives me. He gets em right.
1937		D. Fuchs Low Company 14: According to special information supplied Joe (Grey Lag) Wack by his private clockers, the filly had twice broken thirty-five seconds for the three-eights.
1949		N. Algren Man with the Golden Arm 166: I had to see a clocker. He gave me a good thing for Tropical tomorrow.
1956		‘Toney Betts’ Across the Board 51: There was a clocker’s knock against the horse, who was touched in the wind.

2. (UK Und.) a second-hand car dealer who illegally alters a car’s mileage.

1999

Indep. 23 July 5: The industry is aware that the ‘clockers’ who are caught form the tip of a very large iceberg.

3. (drugs) a dealer of crack cocaine [the need for the drug and the appearances of the dealer both seem to occur at regular intervals, and these dealers are on call ‘around the clock’, but note clock v.² (2)].

1992		R. Price Clockers 19: They didn’t like to snatch clockers later than ten and risk getting stuck [...] with paperwork.
2001		ONDCP Street Terms 6: Clocker — Entry level crack dealers who sell drugs 24 hours a day.