To put in the peg used by climbers (5)
I believe the answer is:
piton
'to put in the peg used by climbers' is the definition.
I can't judge whether this defines the answer.
'to put in the peg' is the wordplay.
'put in' is an insertion indicator.
'the peg' becomes 'pin' ('pin' can be a synonym of 'peg').
'to' put within 'pin' is 'PITON'.
Can you help me to learn more?
(Other definitions for piton that I've seen before include "In top (anag.)" , "What mountaineer requires" , "Peg driven into rock to aid climbing" , "Peg driven into rock in mountain climbing" , "Climber's tool" .)