The most common and most likely answer to this crossword clue is the 9 letter word DRUNKENLY
I've seen this crossword clue in The Guardian
D
R
U
N
K
E
N
L
Y
Buddy explains!
The straight part
We know that "his normal manner of behaving" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case his normal manner of behaving = drunkenly.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.
What does DRUNKENLY mean?
DRUNKENLY adverb- In a drunken manner