The most common and most likely answer to this crossword clue is the 6 plus 3 letter word ATTRIT (+)
I've seen this crossword clue in The Guardian
A
T
T
R
I
T
(
+
)
Buddy explains!

The straight part
We know that "Wear down" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case wear down = attrit (+).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 ATTRIT (+) mean?
ATTRIT (+) noun- One who voluntarily or involuntarily leaves a company; a termed employee
- To wear down through attrition, especially mechanical attrition
- To engage in attrition; to quit or drop out
- To be reduced in quantity through attrition
- To lose, or to kill, troops by attrition due to sustained firepower