The most common and most likely answer to this crossword clue is the 10 letter word PARTICIPLE
I've seen this crossword clue in The Guardian
P
A
R
T
I
C
I
P
L
E
Buddy explains!
The straight part
We know that "grammatical form" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case grammatical form = participle.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 PARTICIPLE mean?
PARTICIPLE noun- (grammar) A form of a verb that may function as an adjective or noun. English has two types of participles: the present participle and the past participle. In other languages, there are others, such as future, perfect, and future perfect participles.