The most common and most likely answer to this crossword clue is the 9 letter word ATTRITION
I've seen this crossword clue in The Times
A
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More clues leading to the result ATTRITION
- Aristocrat losing head’s spoken of natural wastage
- I train tot badly — it’s wearing
- Appeal to drop the act and admit it is harassment …
- Grinding rock into art with this, oddly
- Gradual decrease in hands?
- Some sorrow at sea god seizing island
- Wearing down, turning it into art
- Gradual loss of charm's heartbreaking for one
- Wearing down a primarily tough criminal lacking remorse
- At unending tour, not one falling or wearing down
Buddy explains!

The straight part
We know that "it’s wearing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’s wearing = attrition.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 ATTRITION mean?
ATTRITION noun- Wearing or grinding down by friction.
- The gradual reduction in a tangible or intangible resource due to causes that are passive and do not involve productive use of the resource.
- (human resources) A gradual, natural reduction in membership or personnel, as through retirement, resignation, or death.
- The loss of participants during an experiment.
- Imperfect contrition or remorse.
- The wearing of teeth due to their grinding.
- The loss of a first or second language or a portion of that language.
- To grind or wear down through friction.
- To reduce the number of (jobs or workers) by not hiring new employees to fill positions that become vacant (often with out).
- To undergo a reduction in number.