The most common and most likely answer to this crossword clue is the 7 letter word HOLDERS
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More clues leading to the result HOLDERS
Buddy explains!

The straight part
We know that "proprietors" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case proprietors = holders.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 HOLDERS mean?
HOLDERS noun- A thing that holds.
- A person who temporarily or permanently possesses something.
- One who is employed in the hold of a vessel.
- The defending champion.