The most common and most likely answer to this crossword clue is the 8 letter word BRIGADES
I've seen this crossword clue in The Times
B
R
I
G
A
D
E
S
Buddy explains!

The straight part
We know that "parts of the army" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case parts of the army = brigades.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 BRIGADES mean?
BRIGADES noun- A group of people organized for a common purpose.
- Military unit composed of several regiments (or battalions) and including soldiers from different arms of service.
- A group of people who share views or beliefs.