The most common and most likely answer to this crossword clue is the 8 letter word REJECTED
I've seen this crossword clue in The Guardian
R
E
J
E
C
T
E
D
Buddy explains!

The straight part
We know that "cast aside" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cast aside = rejected.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 REJECTED mean?
REJECTED verb- To refuse to accept.
- To block a shot, especially if it sends the ball off the court.
- To refuse a romantic advance.