The most common and most likely answer to this crossword clue is the 9 letter word FIRSTBORN
I've seen this crossword clue in The Times
F
I
R
S
T
B
O
R
N
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 = firstborn.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 FIRSTBORN mean?
FIRSTBORN noun- The first child to be born to a parent or family.
- Born as the first one in a family, flock or the like.
- Most excellent; most distinguished or exalted.