The most common and most likely answer to this crossword clue is the 4 letter word OLID
I've seen this crossword clue in The Guardian
O
L
I
D
More clues leading to the result OLID
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 = olid.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 OLID mean?
OLID adjective- Evil-smelling; fetid.