The most common and most likely answer to this crossword clue is the 9 letter word POMPOUSLY
I've seen this crossword clue in The Daily Telegraph
P
O
M
P
O
U
S
L
Y
Buddy explains!
The straight part
We know that "in grand manner" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in grand manner = pompously.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.