The most common and most likely answer to this crossword clue is the 6 letter word SKIERS
I've seen this crossword clue in The Daily Telegraph
S
K
I
E
R
S
More clues leading to the result SKIERS
- They send up runners to welcome leading lady
- Sportsmen on mountains, right in their heaven
- Captains ignoring very quiet competitors in winter sport?
- They take terrible risks, taking on ultimate incline
- Penniless captains – they can go downhill fast
- Lousy kisser finding many going down
- Might they risk slaloming round the top of Everest on Sunday?
- Captains not quiet at all as they go downhill
- Sloppy kisser who may go downhill rapidly
- Those sliding on snow risk crashing around end of the slope without leap
Buddy explains!

The straight part
We know that "In cold weather they go downhill fast" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in cold weather they go downhill fast = skiers.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 SKIERS mean?
SKIERS noun- One who skis.
- A ball hit high in the air, often leading to a catch.