The most common and most likely answer to this crossword clue is the 9 letter word DE-STRESS
I've seen this crossword clue in The Guardian
D
E
-
S
T
R
E
S
S
Buddy explains!

The straight part
We know that "Relax" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case relax = de-stress.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 DE-STRESS mean?
DE-STRESS verb- To reduce the stresses in a material.
- To reduce the stress in oneself or another person.
- To reduce emphasis.