The most common and most likely answer to this crossword clue is the 8 letter word DIALYSIS
I've seen this crossword clue in The Times
D
I
A
L
Y
S
I
S
More clues leading to the result DIALYSIS
Buddy explains!

The straight part
We know that "treatment" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case treatment = dialysis.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 DIALYSIS mean?
DIALYSIS noun- A method of separating molecules or particles of different sizes by differential diffusion through a semipermeable membrane.
- Utilization of this method for removal of waste products from the blood in the case of kidney failure: hemodialysis or peritoneal dialysis.
- The spelling out of alternatives, or presenting of either-or arguments that lead to a conclusion.
- Asyndeton.