The most common and most likely answer to this crossword clue is the 9 letter word JANISSARY
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The straight part
We know that "Old soldier" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old soldier = janissary.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 JANISSARY mean?
JANISSARY noun- An infantry soldier, often of Christian descent and forcibly converted to Islam, in a former elite Turkish (Ottoman) guard (disbanded in 1826); by extension, any Turkish soldier, particularly one escorting a traveller.
- An elite, highly loyal supporter.