The most common and most likely answer to this crossword clue is the 10 letter word ASCENDANCY
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Buddy explains!

The straight part
We know that "the highest position" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case the highest position = ascendancy.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 ASCENDANCY mean?
ASCENDANCY noun- The process or period of one's ascent
- Supremacy; dominant control; the quality of being in the ascendant
- A class of Protestant landowners and professionals that dominated political and social life in Ireland up to the early 20th century