The most common and most likely answer to this crossword clue is the 12 letter word HAIRDRESSING
I've seen this crossword clue in Independent
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More clues leading to the result HAIRDRESSING
Buddy explains!

The straight part
We know that "shock treatment" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case shock treatment = hairdressing.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 HAIRDRESSING mean?
HAIRDRESSING verb- To dress or style hair.
- The washing, colouring, cutting and styling of the hair; the art or trade of a hairdresser.