The most common and most likely answer to this crossword clue is the 3 plus 3 plus 5 plus 4 letter word HIT THE RIGHT NOTE
I've seen this crossword clue in The Times
H
I
T
T
H
E
R
I
G
H
T
N
O
T
E
Buddy explains!
The straight part
We know that "to behave appropriately" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case to behave appropriately = hit the right note.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.