The most common and most likely answer to this crossword clue is the 2 plus 4 plus 2 plus 4 letter word AS GOOD AS GOLD
I've seen this crossword clue in Independent
A
S
G
O
O
D
A
S
G
O
L
D
More clues leading to the result AS GOOD AS GOLD
Buddy explains!
The straight part
We know that "with perfect manners" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case with perfect manners = as good as gold.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.