The most common and most likely answer to this crossword clue is the 2 plus 4 plus 6 letter word ON ONES UPPERS
I've seen this crossword clue in Financial Times
O
N
O
N
E
S
U
P
P
E
R
S
More clues leading to the result ON ONES UPPERS
Buddy explains!

The straight part
We know that "Where to rub 9/skint" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case where to rub 9/skint = on ones uppers.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.