The most common and most likely answer to this crossword clue is the 6 letter word BREVET
B
R
E
V
E
T
Buddy explains!
The straight part
We know that "R/military promotion" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case r/military promotion = brevet.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 BREVET mean?
BREVET noun- A military document entitling a commissioned officer to hold a higher rank temporarily, but without an increase in pay.
- A warrant from the government, granting a privilege, title, or dignity, as in France.
- An organized, long-distance bicycle ride — not a race, but a test of endurance — which follows a designated but unmarked route passing through check points.
- To promote by brevet.