The most common and most likely answer to this crossword clue is the 9 letter word TRAINBAND
I've seen this crossword clue in The Times
T
R
A
I
N
B
A
N
D
More clues leading to the result TRAINBAND
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The straight part
We know that "old citizen soldiers" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old citizen soldiers = trainband.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 TRAINBAND mean?
TRAINBAND noun- A company of trained civilian militia operating in England and North America between the 16th and the 18th centuries.