The most common and most likely answer to this crossword clue is the 9 plus 3 letter word BUNDESTAG TAG
I've seen this crossword clue in Independent
B
U
N
D
E
S
T
A
G
T
A
G
Buddy explains!
The straight part
We know that "German parliament building" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case german parliament building = bundestag tag.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.