The most common and most likely answer to this crossword clue is the 11 letter word BLING-BLING
I've seen this crossword clue in The Times
B
L
I
N
G
-
B
L
I
N
G
Buddy explains!
The straight part
We know that "Flash" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case flash = bling-bling.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 BLING-BLING mean?
BLING-BLING noun- Shiny jewelry that displays wealth, such as a diamond ring or a stylish gold necklace or bracelet.