The most common and most likely answer to this crossword clue is the 9 letter word POSTERITY
I've seen this crossword clue in The Guardian
P
O
S
T
E
R
I
T
Y
Buddy explains!
The straight part
We know that "future generations" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case future generations = posterity.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 POSTERITY mean?
POSTERITY noun- All the future generations, especially the descendants of a specific person.