But a climber really needs more than one! (4)
I believe the answer is:
limb
'one' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'but a climber really needs more than' is the wordplay.
'LIMB' is hidden within the letters.
However, I'm uncertain how the hidden word is indicated.
Can you help me to learn more?
(Other definitions for limb that I've seen before include "Branch - member" , "Substantial branch from a tree trunk" , "as part of plane, say" , "Arm, leg or wing" , "Eg wing" .)