The most common and most likely answer to this crossword clue is the 17 letter word SEMANTRA/TRABEATE
S
E
M
A
N
T
R
A
/
T
R
A
B
E
A
T
E
Buddy explains!
The straight part
We know that "built of horizontal beams" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case built of horizontal beams = semantra/trabeate.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.