The most common and most likely answer to this crossword clue is the 4 letter word LAZE
I've seen this crossword clue in The Daily Telegraph
L
A
Z
E
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 = laze.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 LAZE mean?
LAZE noun- An instance of lazing.
- Laziness.
- To be lazy, waste time.
- To pass time relaxing; to relax, lounge.
- Acidic steam created when super-hot lava contacts salt water.