The most common and most likely answer to this crossword clue is the 6 letter word ABRADE
I've seen this crossword clue in The Times
A
B
R
A
D
E
More clues leading to the result ABRADE
Buddy explains!

The straight part
We know that "Rub" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rub = abrade.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 ABRADE mean?
ABRADE verb- To rub or wear off; erode.
- To wear down or exhaust, as a person; irritate.
- To irritate by rubbing; chafe.
- To cause the surface to become more rough.
- To undergo abrasion.
- To wrench (something) out.
- To wake up.
- To spring, start, make a sudden movement.
- To shout out.
- To rise in the stomach with nausea.