The most common and most likely answer to this crossword clue is the 3 plus 2 plus 5 plus 5 letter word OUT OF WHOLE CLOTH
O
U
T
O
F
W
H
O
L
E
C
L
O
T
H
Buddy explains!
The straight part
We know that "entirely from fantasy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case entirely from fantasy = out of whole cloth.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.