The most common and most likely answer to this crossword clue is the 13 letter word SPLENDIFEROUS
I've seen this crossword clue in The Guardian
S
P
L
E
N
D
I
F
E
R
O
U
S
More clues leading to the result SPLENDIFEROUS
Buddy explains!
The straight part
We know that "Fantastically fine and super, so extremely delightful" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fantastically fine and super, so extremely delightful = splendiferous.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 SPLENDIFEROUS mean?
SPLENDIFEROUS adjective- Beautiful, splendid