The most common and most likely answer to this crossword clue is the 12 letter word BRAINWASHING
I've seen this crossword clue in The Daily Telegraph
B
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More clues leading to the result BRAINWASHING
Buddy explains!
The straight part
We know that "Clearing one’s mind" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case clearing one’s mind = brainwashing.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 BRAINWASHING mean?
BRAINWASHING noun- A form of indoctrination that forces people to abandon their beliefs in favour of another set of beliefs by conditioning through various forms of pressure or torture
- To affect one's mind by using extreme mental pressure or any other mind-affecting process. (e.g. hypnosis)
- To take from an electronically controlled machine its stored-up information; to erase a computer's programming. (1960)