The most common and most likely answer to this crossword clue is the 9 letter word LEGIONARY
I've seen this crossword clue in The Daily Telegraph
L
E
G
I
O
N
A
R
Y
Buddy explains!
The straight part
We know that "soldier long ago" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case soldier long ago = legionary.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 LEGIONARY mean?
LEGIONARY noun- A soldier belonging to a legion; a professional soldier of the ancient Roman army.
- A member of a legion, such as the American Legion, or of any organization containing the term legion in its title (e.g. the French Foreign Legion).
- Relating to, or consisting of, a legion or legions.
- Containing a great number.