The most common and most likely answer to this crossword clue is the 8 plus 3 letter word CRIMINAL LAW
I've seen this crossword clue in The Times
C
R
I
M
I
N
A
L
L
A
W
More clues leading to the result CRIMINAL LAW
Buddy explains!

The straight part
We know that "that’s used in constructing case" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case that’s used in constructing case = criminal law.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.