The most common and most likely answer to this crossword clue is the 4 letter word MOLY
I've seen this crossword clue in Financial Times
M
O
L
Y
Buddy explains!
The straight part
We know that "fabulous herb" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fabulous herb = moly.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 MOLY mean?
MOLY noun- A magic herb or plant used by Odysseus to overcome Circe.
- Any plant associated with the mythological moly, especially the European allium, Allium moly.
- Molybdenum
- Molybdenum grease