Supple mountaineer ignoring cold at first (6)

I believe the answer is:

limber

I'm a little stuck... Click here to teach me more about this clue!

'supple' is the definition.
(I know that supple can be written as limber)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for limber that I've seen before include "Lithe" , "Supple; flexible" , "Agile and flexible" , "Agile, supple" , "Able to move supply" .)

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