The most common and most likely answer to this crossword clue is the 4 plus 6 plus 4 letter word TAKE THINGS EASY
I've seen this crossword clue in The Daily Telegraph
T
A
K
E
T
H
I
N
G
S
E
A
S
Y
More clues leading to the result TAKE THINGS EASY
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 = take things easy.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.