The most common and most likely answer to this crossword clue is the 10 letter word SERVICEMAN
I've seen this crossword clue in The Daily Telegraph
S
E
R
V
I
C
E
M
A
N
More clues leading to the result SERVICEMAN
Buddy explains!
The straight part
We know that "one in the armed forces" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one in the armed forces = serviceman.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 SERVICEMAN mean?
SERVICEMAN noun- A man who serves in the armed forces.
- A person, especially a man, employed in service and maintenance of equipment.