The most common and most likely answer to this crossword clue is the 4 plus 4 plus 2 plus 3 plus 2 letter word MIND ONES PS AND QS
I've seen this crossword clue in The Daily Telegraph
M
I
N
D
O
N
E
S
P
S
A
N
D
Q
S
Buddy explains!
The straight part
We know that "Be polite" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case be polite = mind ones ps and qs.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.