The most common and most likely answer to this crossword clue is the 12 letter word HYPNOTHERAPY
I've seen this crossword clue in The Daily Telegraph
H
Y
P
N
O
T
H
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A
P
Y
More clues leading to the result HYPNOTHERAPY
Buddy explains!

The straight part
We know that "course of treatment" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case course of treatment = hypnotherapy.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 HYPNOTHERAPY mean?
HYPNOTHERAPY noun- Treatment of disease by means of hypnotism.