The most common and most likely answer to this crossword clue is the 6 letter word OGIVES
O
G
I
V
E
S
Buddy explains!

The straight part
We know that "building arches" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case building arches = ogives.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.
What does OGIVES mean?
OGIVES noun- The curve of a cumulative distribution function.
- A Gothic pointed arch, or a rib of a Gothic vault.
- The pointed, curved nose of a bullet, missile, or rocket.
- A three-dimensional wave-bulge, characteristic of glaciers that have experienced extreme underlying topographic change.