The most common and most likely answer to this crossword clue is the 3 plus 5 plus 5 plus 2 plus 3 plus 3 plus 3 plus 6 letter word YOU CAN’T TEACH AN OLD DOG NEW TRICKS
I've seen this crossword clue in The Guardian
Y
O
U
C
A
N
â
€
™
T
T
E
A
C
H
A
N
O
L
D
D
O
G
N
E
W
T
R
I
C
K
S
Buddy explains!
The straight part
We know that "impossible to change the mindset" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case impossible to change the mindset = you can’t teach an old dog new tricks.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.