
1) An official who times a race 2) Former Amtrak route 3) Minimalistic composition
Found on
https://www.crosswordclues.com/clue/clocker

Clocker, for clock, galvanic skin response sensor and digital delay system, is a minimalist electronic music piece by Alvin Lucier conceived in 1978, though Lucier felt there did not exist an appropriate digital delay system till 1988: `With this new equipment, the sounds of the delayed clock now matched those of the original, creating clear copi....
Found on
http://en.wikipedia.org/wiki/Clocker
[train] Clockers were regular passenger trains between Philadelphia and New York City on the Northeast Corridor; on the Pennsylvania Railroad they left each terminal on the hour until the 1950s and a less-regular schedule continued on Amtrak. The first train run by Amtrak was a Clocker that left Penn Station at 12:05 AM on May 1, 1971. The ...
Found on
http://en.wikipedia.org/wiki/Clocker_(train)

a person who times racehorses during tryouts to determine their speed. · an official who times a race. · a person who maintains a record of the flow of traffic, as of visitors to a museum.
Found on
https://www.infoplease.com/dictionary/clocker
No exact match found.