The most common and most likely answer to this crossword clue is the 4 plus 2 plus 7 letter word MAKE NO MISTAKE
I've seen this crossword clue in The Daily Telegraph
M
A
K
E
N
O
M
I
S
T
A
K
E
Buddy explains!
The straight part
We know that "Get nothing wrong/Don’t be deceived into thinking otherwise" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case get nothing wrong/don’t be deceived into thinking otherwise = make no mistake.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.