The most common and most likely answer to this crossword clue is the 5 plus 4 letter word STEAM BATH
I've seen this crossword clue in The Guardian
S
T
E
A
M
B
A
T
H
More clues leading to the result STEAM BATH
Buddy explains!
The straight part
We know that "relaxing treatment" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case relaxing treatment = steam bath.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.