The most common and most likely answer to this crossword clue is the 11 letter word ATTRIBUTIVE
I've seen this crossword clue in Independent
A
T
T
R
I
B
U
T
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E
Buddy explains!

The straight part
We know that "adding something grammatically" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case adding something grammatically = attributive.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.
Crossword Answer definitions
ATTRIBUTIVE noun- (grammar) An attributive word or phrase (see above), contrasted with predicative or substantive.
- (grammar, of a word or phrase) Modifying a noun, while in the same phrase as that noun.
- Having the nature of an attribute.
- Serving to express an attribute of an object.