The most common and most likely answer to this crossword clue is the 3 plus 2 plus 3 plus 1 plus 6 letter word AIR ON THE G STRING
I've seen this crossword clue in The Guardian
A
I
R
O
N
T
H
E
G
S
T
R
I
N
G
Buddy explains!
The straight part
We know that "a German number" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a german number = air on the g string.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.