The most common and most likely answer to this crossword clue is the 3 plus 7 plus 5 letter word SAN ANDREAS FAULT
I've seen this crossword clue in The Sunday Telegraph
S
A
N
A
N
D
R
E
A
S
F
A
U
L
T
More clues leading to the result SAN ANDREAS FAULT
- This, under LA, SF and Santa A, in slippage?
- Dicky lands a tuna: fears line gradually slipping
- Shocking and unsafe: LA star moving houses over this?
- Safe and natural cracks under capital in Sacramento, this crack potentially unsafe
- Fan ran and assaulted drunk in the break in America
- Unexpected turn, as a sand flea exposes geological feature
- Deep split might cause the downfall of Hollywood
- Digger initially going in faster, as annual repairs concerning underground line?
Buddy explains!
The straight part
We know that "underground line?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case underground line? = san andreas fault.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.