The most common and most likely answer to this crossword clue is the 13 letter word HIGH-SOUNDING
I've seen this crossword clue in The Times
H
I
G
H
-
S
O
U
N
D
I
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G
Buddy explains!
The straight part
We know that "Grandiose" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case grandiose = high-sounding.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 HIGH-SOUNDING mean?
HIGH-SOUNDING adjective- Having an elevated or moralistic tone; pompous.
- Oratorial, sometimes to the point of sounding contrived.