Mountaineering aid to fasten all round (5)
I believe the answer is:
piton
'mountaineering aid' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'to fasten all round' is the wordplay.
'fasten' becomes 'pin' (pinning is a kind of fastening).
'all round' indicates putting letters inside.
'to' inserted within 'pin' is 'PITON'.
Can you help me to learn more?
(Other definitions for piton that I've seen before include "Peg driven into rock as climbing support" , "Climber's iron peg" , "something upwardly mobile type hammers home" , "Metal peg used in rock climbing" , "Climber's spike" .)