The most common and most likely answer to this crossword clue is the 9 letter word MENTALITY
I've seen this crossword clue in The Daily Telegraph
M
E
N
T
A
L
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T
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More clues leading to the result MENTALITY
Buddy explains!

The straight part
We know that "way of thinking" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case way of thinking = mentality.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 MENTALITY mean?
MENTALITY noun- A mindset; a way of thinking; a set of beliefs.
- The characteristics of a mind described as a system of distinctive structures and processes based in biology, language, or culture, etc.; a mental system.