The most common and most likely answer to this crossword clue is the 9 letter word REGIMENTS
I've seen this crossword clue in The Times
R
E
G
I
M
E
N
T
S
Buddy explains!
The straight part
We know that "army units" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case army units = regiments.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 REGIMENTS mean?
REGIMENTS noun- A unit of armed troops under the command of an officer, and consisting of several smaller units; now specifically, usually composed of two or more battalions.
- Rule or governance over a person, place etc.; government, authority.
- The state or office of a ruler; rulership.
- Influence or control exercised by someone or something (especially a planet).
- A place under a particular rule; a kingdom or domain.
- A regimen.
- To form soldiers into a regiment.
- To systematize, or put in rigid order.