The most common and most likely answer to this crossword clue is the 7 plus 9 letter word CAPTAIN (ABSTAIN)
I've seen this crossword clue in The Guardian
C
A
P
T
A
I
N
(
A
B
S
T
A
I
N
)
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 = captain (abstain).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.