The most common and most likely answer to this crossword clue is the 6 letter word SMELLY
I've seen this crossword clue in Financial Times
S
M
E
L
L
Y
More clues leading to the result SMELLY
Buddy explains!

The straight part
We know that "rank" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rank = smelly.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 SMELLY mean?
SMELLY adjective- Having a bad smell.
- Having a quality that arouses suspicion.
- (in extreme programming) Having signs that suggest a design problem; having a code smell.
- A Short Magazine Lee Enfield rifle or one of its derivatives.