The most common and most likely answer to this crossword clue is the 10 letter word MARTINGALE
I've seen this crossword clue in The Daily Telegraph
M
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Buddy explains!
The straight part
We know that "part of harness" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case part of harness = martingale.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 MARTINGALE mean?
MARTINGALE noun- A piece of harness used on a horse to keep it from raising its head above a desired point.
- A spar, or piece of rigging that strengthens the bowsprit.
- A stochastic process for which the conditional expectation of future values given the sequence of all prior values is equal to the current value.
- A gambling strategy in which one doubles the stake after each loss.
- A strap attached to the sword handle, preventing a sword being dropped if disarmed.
- To employ the martingale strategy in gambling.