The most common and most likely answer to this crossword clue is the 10 letter word FIRST-BORN
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More clues leading to the result FIRST-BORN
Buddy explains!

The straight part
We know that "Oldest" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case oldest = first-born.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 FIRST-BORN mean?
FIRST-BORN adjective- Born as the first one in a family, flock or the like.
- Most excellent; most distinguished or exalted.