Climber's spike (5)
I believe the answer is:
piton
'climber's spike' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for piton that I've seen before include "Climber's spike" , "Metal spike used in rock climbing" , "Peg driven into rock in mountain climbing" , "Climber's iron peg" , "What mountaineer requires" .)