The most common and most likely answer to this crossword clue is the 10 letter word INEXORABLE
I've seen this crossword clue in The Times
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More clues leading to the result INEXORABLE
- Relentless old men with capacity to follow batting?
- The Spanish port in Italy filled with cattle being raised is irresistible
- Relentless old men with capacity to support home
- Unstoppable boxer, Ali, and George Foreman ultimately in rumble
- One’s cattle returned half-heartedly, herd not to be moved?
- Unpreventable ban or exile abroad
Buddy explains!

The straight part
We know that "Relentless" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case relentless = inexorable.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 INEXORABLE mean?
INEXORABLE adjective- Impossible to prevent or stop; inevitable.
- Unable to be persuaded; relentless; unrelenting.
- Adamant; severe.