Mountaineers, supple, in Caucasus on vacation (8)
I believe the answer is:
climbers
'mountaineers' is the definition.
(I know this)
'supple in caucasus on vacation' is the wordplay.
'supple' becomes 'limber' (synonyms).
'in' is an insertion indicator.
'on vacation' suggests removing the centre.
'caucasus' with its middle removed is 'cs'.
'limber' placed inside 'cs' is 'CLIMBERS'.
(Other definitions for climbers that I've seen before include "Plants that grow up a support" , "Edmund Hillary and Tensing Norgay, say" , "Edmund Hillary and Reinhold Messner, say" .)