The most common and most likely answer to this crossword clue is the 6 letter word LIMBER
I've seen this crossword clue in The Guardian
L
I
M
B
E
R
More clues leading to the result LIMBER
- Supple mountaineer ignoring cold at first
- Mountaineer lost his head but remained flexible
- Able to move supply and perhaps arm monarch
- Flexible member joining ends of extensor
- Graceful mountaineer’s heading off
- Agile mountaineer, knocking top off
- Ignoring cold, mountaineer is able to move freely
- Slim Bermudan’s somewhat flexible
Buddy explains!

The straight part
We know that "Supple" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case supple = limber.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 LIMBER mean?
LIMBER verb- To cause to become limber; to make flexible or pliant.
- Flexible, pliant, bendable.
- A two-wheeled vehicle to which a wheeled artillery piece or caisson may be attached for transport.
- (in the plural) The shafts or thills of a wagon or carriage.
- (in the plural) Gutters or conduits on each side of the keelson to allow water to pass to the pump well.
- To prepare an artillery piece for transportation (i.e., to attach it to its limber.)