The most common and most likely answer to this crossword clue is the 6 plus 3 plus 5 letter word GEORGE THE FIRST
I've seen this crossword clue in The Sunday Telegraph
G
E
O
R
G
E
T
H
E
F
I
R
S
T
Buddy explains!
The straight part
We know that "German king" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case german king = george the first.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.