The most common and most likely answer to this crossword clue is the 6 letter word CERIPH
C
E
R
I
P
H
Buddy explains!
The straight part
We know that "Letter’s rare flourish" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case letter’s rare flourish = ceriph.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.