A job to hold back horse, keeping its head turning away from the ordinary course (10)
I believe the answer is:
apostrophe
'a job' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for apostrophe that I've seen before include "greengrocer abuses it!)" , "At which point discarded letters" , "Sign of omission" , "Often misplaced punctuation mark" , "Passage addressed to character" .)