The most common and most likely answer to this crossword clue is the 5 plus 5 letter word CRASH DIVES
I've seen this crossword clue in The Guardian
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Buddy explains!
The straight part
We know that "Plunges steeply" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case plunges steeply = crash dives.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 CRASH DIVES mean?
CRASH DIVES noun- An emergency maneuver by a submarine in which it suddenly descends, especially to considerable depth, usually in order to escape an attack or to avoid a collision.
- A rapid descent by a person or thing ending in a violent collision.
- (often hyphenated, submarine operation) To perform a crash dive.
- (often hyphenated, submarine operation) To cause (a submarine) to perform a crash dive.
- (often hyphenated) To rapidly descend, intentionally or accidentally, in a manner that ends in a violent collision.
- (often hyphenated) To rapidly descend, in a manner that ends in a violent collision with (something).
- (often hyphenated) To cause (something) to descend rapidly, in a manner that ends in a violent collision.