The most common and most likely answer to this crossword clue is the 8 letter word PRESIDIO
I've seen this crossword clue in The Times
P
R
E
S
I
D
I
O
More clues leading to the result PRESIDIO
- Period is reviewed in military post
- Periods I spent in fort
- Before man lands on the moon, make settlement in Spain
- Military post is set up in troubled period
- Spanish fortress where, initially, Public Relations Information Officer went around after reorganising side
- Rio spied beleaguered fort
- Live mostly in very good old military post
Buddy explains!

The straight part
We know that "military post" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case military post = presidio.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 PRESIDIO mean?
PRESIDIO noun- A garrisoned place, especially one that is or was once under Spanish control.