The most common and most likely answer to this crossword clue is the 9 letter word GARRISONS
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The straight part
We know that "military establishments" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case military establishments = garrisons.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 GARRISONS mean?
GARRISONS noun- A permanent military post.
- The troops stationed at such a post.
- (allusive) Occupants.
- (U.S. Space Force) A military unit, nominally headed by a colonel, equivalent to a USAF support wing, or an army regiment.