The most common and most likely answer to this crossword clue is the 8 letter word RELAXING
I've seen this crossword clue in The Guardian
R
E
L
A
X
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G
More clues leading to the result RELAXING
Buddy explains!

The straight part
We know that "at rest" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case at rest = relaxing.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 RELAXING mean?
RELAXING verb- To calm down.
- To make something loose.
- To become loose.
- To make something less severe or tense.
- To become less severe or tense.
- To make something (such as codes and regulations) more lenient.
- (of codes and regulations) To become more lenient.
- To relieve (something) from stress.
- To relieve from constipation; to loosen; to open.
- The process by which something is relaxed.
- Conducive to relaxation; helping one to relax.