The most common and most likely answer to this crossword clue is the 11 letter word SNOWBOARDER
I've seen this crossword clue in The Times
S
N
O
W
B
O
A
R
D
E
R
Buddy explains!
The straight part
We know that "one heading downhill fast" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one heading downhill fast = snowboarder.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.