The most common and most likely answer to this crossword clue is the 8 letter word TROTTING
I've seen this crossword clue in The Daily Telegraph
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More clues leading to the result TROTTING
Buddy explains!
The straight part
We know that "harness racing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case harness racing = trotting.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does TROTTING mean?
TROTTING verb- To move along briskly; specifically, to move at a pace between a walk and a run.
- (of a horse) To move at a gait between a walk and a canter.
- To cause to move, as a horse or other animal, in the pace called a trot; to cause to run without galloping or cantering.
- The action of the verb trot.
- Of an animal that trots.