The most common and most likely answer to this crossword clue is the 3 plus 8 plus 2 letter word LET YOURSELF GO
I've seen this crossword clue in The Guardian
L
E
T
Y
O
U
R
S
E
L
F
G
O
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 = let yourself go.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.