The most common and most likely answer to this crossword clue is the 5 plus 6 letter word CRASH HELMET
I've seen this crossword clue in The Times
C
R
A
S
H
H
E
L
M
E
T
Buddy explains!
The straight part
We know that "case a nut goes inside" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case case a nut goes inside = crash helmet.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.