The most common and most likely answer to this crossword clue is the 8 letter word MONOBLOC
I've seen this crossword clue in The Times
M
O
N
O
B
L
O
C
Buddy explains!
The straight part
We know that "Cast in one" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cast in one = monobloc.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 MONOBLOC mean?
MONOBLOC noun- A forging or casting made in a single piece, rather than being fabricated from components