The most common and most likely answer to this crossword clue is the 9 letter word SOLDIERED
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More clues leading to the result SOLDIERED
Buddy explains!

The straight part
We know that "served in the army" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case served in the army = soldiered.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 SOLDIERED mean?
SOLDIERED verb- To continue steadfast; to keep striving.
- To serve as a soldier.
- To intentionally restrict labor productivity; to work at the slowest rate that goes unpunished.
- To take a ride on (another person's horse) without permission.