Cryptic crossword clue
In all seriousness, Jet’s leader sounds good in good American role 6,5

The most common and most likely answer to this crossword clue is the 6 plus 5 letter word JOKING APART

I've seen this crossword clue in The Daily Telegraph
J
O
K
I
N
G
A
P
A
R
T

Buddy explains!

The straight part
We know that "In all seriousness" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in all seriousness = joking apart.
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.

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