The most common and most likely answer to this crossword clue is the 9 letter word ONRUSHING
I've seen this crossword clue in The Daily Telegraph
O
N
R
U
S
H
I
N
G
Buddy explains!
The straight part
We know that "approaching at speed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case approaching at speed = onrushing.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 ONRUSHING mean?
ONRUSHING verb- To rush or flow forward forcefully.
- To assault aggressively.
- Rushing or flowing forward
- Attacking