The most common and most likely answer to this crossword clue is the 9 plus 3 plus 5 letter word SWEETNESS AND LIGHT
I've seen this crossword clue in The Times
S
W
E
E
T
N
E
S
S
A
N
D
L
I
G
H
T
Buddy explains!
The straight part
We know that "pleasant manner" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case pleasant manner = sweetness and light.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.