The most common and most likely answer to this crossword clue is the 8 letter word STEEP-TO
S
T
E
E
P
-
T
O
Buddy explains!
The straight part
We know that "plunging straight down" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case plunging straight down = steep-to.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.