You can see them playing a part in mountain ascents! (5)
I believe the answer is:
limbs
'you can see them playing a' is the definition.
I can't tell whether this defines the answer.
'part in mountain ascents' is the wordplay.
'part' becomes 'limb' (limb is a kind of part).
'in mountain ascents' becomes 's' (I can't explain this - if you can you should give a lot more credence to this answer).
'limb'+'s'='LIMBS'
Can you help me to learn more?
(Other definitions for limbs that I've seen before include "legs, perhaps" , "Arms and legs" .)