The most common and most likely answer to this crossword clue is the 10 letter word ACQUAINTED
I've seen this crossword clue in The Guardian
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More clues leading to the result ACQUAINTED
- Familiar expert making case for sweet drinks primarily
- Friendly and well served with charming interior
- Virtuoso keeps old-fashioned duke on friendly terms
- Familiar expert entertaining charmingly old-fashioned daughter
- Familiar sweet filling maestro put on starter for dinner
- Familiar kind of service entertains twee daughter
- Playing card incorporating old-fashioned diamonds is familiar
- Informed expert about charmingly old-fashioned duke
- Familiar with Bill, olde-worlde journalist
- Familiar Conservative, in blue, mean to dismiss knight
Buddy explains!

The straight part
We know that "on friendly terms" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case on friendly terms = acquainted.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 ACQUAINTED mean?
ACQUAINTED verb- (followed by with) To furnish or give experimental knowledge of; to make (one) to know; to make familiar.
- (followed by of or that) To communicate notice to; to inform; to make cognizant.
- To familiarize; to accustom.
- Personally known; familiar.