The most common and most likely answer to this crossword clue is the 4 letter word ERUV
E
R
U
V
More clues leading to the result ERUV
Buddy explains!

The straight part
We know that "Sabbath relaxation" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sabbath relaxation = eruv.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 ERUV mean?
ERUV noun- (most commonly) An eruv chatzerot; ritual aggregation of properties that allows Jews observing traditional Shabbat rules to carry burdens across property lines.
- One of the other types of eruv; an eruv tavshilin or eruv techumin.