The most common and most likely answer to this crossword clue is the 6 letter word HOBBIT
H
O
B
B
I
T
Buddy explains!

The straight part
We know that "Fantastic being" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fantastic being = hobbit.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 HOBBIT mean?
HOBBIT noun- A Welsh unit of weight, equal to four Welsh pecks, or 168 pounds
- An old unit of volume (2 1/2 bushels, the volume of 168 pounds of wheat).