The most common and most likely answer to this crossword clue is the 8 letter word GENITIVE
I've seen this crossword clue in The Daily Telegraph
G
E
N
I
T
I
V
E
Buddy explains!
The straight part
We know that "case" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case case = genitive.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 GENITIVE mean?
GENITIVE noun- (grammar) An inflection pattern (of any given language) that expresses origin or ownership and possession.
- (grammar) A word inflected in the genitive case; a word indicating origin, ownership or possession.
- (grammar) Of or pertaining to that case (as the second case of Latin and Greek nouns) which expresses a quality, origin or possession. It corresponds to the possessive case in English.