The most common and most likely answer to this crossword clue is the 8 letter word CASTLING
I've seen this crossword clue in The Guardian
C
A
S
T
L
I
N
G
Buddy explains!
The straight part
We know that "protecting the king" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case protecting the king = castling.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 CASTLING mean?
CASTLING noun- An abortion, or a premature birth.
- The second or third swarm of bees which leaves a hive in a season.
- A miniature cast or mould.
- One that is cast.
- To house or keep in a castle.
- To protect or separate in a similar way.
- To make into a castle: to build in the form of a castle or add (real or imitation) battlements to an existing building.
- (usually intransitive) To move the king 2 squares right or left and, in the same turn, the nearest rook to the far side of the king. The move now has special rules: the king cannot be in, go through, or end in check; the squares between the king and rook must be vacant; and neither piece may have been moved before castling.
- (usually intransitive) To create a similar defensive position in Japanese chess through several moves.
- To bowl a batsman with a full-length ball or yorker such that the stumps are knocked over.
- A move in which the king moves two squares towards a rook, and the rook moves to the other side of the king; the action of the verb to castle.
- The act of constructing a defense structure in Japanese chess in which the king (玉) is positioned in a certain way so that it is protected by pawns (歩) and silver general(s) (銀) and/or gold general(s) (金) often with an additional knight (桂) and lance (香車).