The most common and most likely answer to this crossword clue is the 10 letter word IMPERATIVE
I've seen this crossword clue in The Daily Telegraph
I
M
P
E
R
A
T
I
V
E
Buddy explains!
The straight part
We know that "Essential/term in grammar" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case essential/term in grammar = imperative.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 IMPERATIVE mean?
IMPERATIVE noun- (grammar) The grammatical mood expressing an order (see jussive). In English, the imperative form of a verb is the same as that of the bare infinitive.
- (grammar) A verb in imperative mood.
- An essential action, a must: something which is imperative.
- Essential; crucial; extremely important.
- (grammar) Of, or relating to the imperative mood.
- Having semantics that incorporates mutable variables.
- Expressing a command; authoritatively or absolutely directive.