The most common and most likely answer to this crossword clue is the 8 plus 10 letter word FRIPPERY (CKNEAPCB)
F
R
I
P
P
E
R
Y
(
C
K
N
E
A
P
C
B
)
Buddy explains!
The straight part
We know that "Old cast-offs" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old cast-offs = frippery (ckneapcb).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.