The most common and most likely answer to this crossword clue is the 6 letter word CASING
I've seen this crossword clue in The Guardian
C
A
S
I
N
G
More clues leading to the result CASING
Buddy explains!

The straight part
We know that "Protection" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case protection = casing.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 CASING mean?
CASING verb- To propose hypothetical cases.
- To place (an item or items of manufacture) into a box, as in preparation for shipment.
- To cover or protect with, or as if with, a case; to enclose.
- To survey (a building or other location) surreptitiously, as in preparation for a robbery.
- That which encloses or encases.
- The decorative trim around a door or window.
- A metal pipe used to line the borehole of a well.
- The collective states of upper and lower case letters.