The most common and most likely answer to this crossword clue is the 12 letter word SPELLBINDING
I've seen this crossword clue in The Daily Telegraph
S
P
E
L
L
B
I
N
D
I
N
G
Buddy explains!

The straight part
We know that "Fascinating" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fascinating = spellbinding.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 SPELLBINDING mean?
SPELLBINDING adjective- Engrossing; fascinating; gaining rapt attention; captivating.
- Having the power to bind magically through the agency of a spell.