Mountaineer's descent needs muscles — after which rest up (6)
I believe the answer is:
abseil
'mountaineer's descent needs muscles' is the definition.
I can't tell whether this definition defines the answer.
'which rest up' is the wordplay.
'which' becomes 'abs' (I can't explain this - if you can you should give a lot more credence to this answer).
'rest' becomes 'lie' (lying is a kind of resting).
'up' is a reversal indicator (in down clue: letters go upwards).
'lie' reversed gives 'eil'.
'abs'+'eil'='ABSEIL'
'after' is the link.
Can you help me to learn more?
(Other definitions for abseil that I've seen before include "Descend using running rope" , "Is able (anag.)" , "Descend rock face on rope" , "Use a rope to climb down a mountain" , "Climb down using a double rope" .)