The most common and most likely answer to this crossword clue is the 10 letter word IMPLACABLE
I've seen this crossword clue in The Daily Telegraph
I
M
P
L
A
C
A
B
L
E
More clues leading to the result IMPLACABLE
- Remorseless climbing mountain chain, I'm at the top!
- Intransigent troublemaker left a message
- Rogue politician suppressing the French is unable to be stopped
- Rogue Hollywood guy is relentless
- Fierce wild mammal missing a chain
- One politician, the French cabinet minister, such as can’t be appeased
- Merciless message by the devil, heart of clay
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 = implacable.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 IMPLACABLE mean?
IMPLACABLE adjective- Not able to be placated or appeased.
- Impossible to prevent or stop; inexorable, unrelenting, unstoppable.
- Adamant; immovable.