The most common and most likely answer to this crossword clue is the 9 letter word ADVERBIAL
I've seen this crossword clue in Financial Times
A
D
V
E
R
B
I
A
L
Buddy explains!

The straight part
We know that "In grammatical terms, well/In grammatical terms, /well" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in grammatical terms, well/in grammatical terms, /well = adverbial.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 ADVERBIAL mean?
ADVERBIAL noun- (grammar) An adverbial word or phrase.
- (grammar) Of or relating to an adverb.