The most common and most likely answer to this crossword clue is the 10 letter word IDEMPOTENT
I've seen this crossword clue in The Guardian
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Buddy explains!

The straight part
We know that "Unchanged by Tim/es/?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unchanged by tim/es/? = idempotent.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 IDEMPOTENT mean?
IDEMPOTENT noun- An idempotent element.
- An idempotent structure.
- Said of a function: describing an action which, when performed multiple times on the same subject, has no further effect on its subject after the first time it is performed.
- Said of an element of an algebraic structure with a binary operation (such as a group or semigroup): when the element operates on itself, the result is equal to itself.
- Said of a binary operation: that all of the distinct elements it can operate on are idempotent (in the sense given just above).
- Said of an algebraic structure: having an idempotent operation (in the sense above).