The most common and most likely answer to this crossword clue is the 12 plus 5 letter word BROADCASTING HOUSE
I've seen this crossword clue in The Times
B
R
O
A
D
C
A
S
T
I
N
G
H
O
U
S
E
Buddy explains!
The straight part
We know that "Art Deco building" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case art deco building = broadcasting house.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.