The most common and most likely answer to this crossword clue is the 5 plus 4 letter word DEATH RATE
I've seen this crossword clue in The Daily Telegraph
D
E
A
T
H
R
A
T
E
More clues leading to the result DEATH RATE
Buddy explains!
The straight part
We know that "The speed at which we go" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case the speed at which we go = death rate.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.