The most common and most likely answer to this crossword clue is the 8 letter word INTRADOS
I've seen this crossword clue in The Guardian
I
N
T
R
A
D
O
S
More clues leading to the result INTRADOS
Buddy explains!

The straight part
We know that "Curve under an arch/Curve under an arch" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case curve under an arch/curve under an arch = intrados.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 INTRADOS mean?
INTRADOS noun- The inner curve of an arch or vault.