Reserve selection that may be used to gain a foothold (3,4)
I believe the answer is:
ice pick
'that may be used to gain a foothold' is the definition.
(tool used when climbing)
'reserve selection' is the wordplay.
'reserve' becomes 'ice' (both can mean aloofness).
'selection' becomes 'pick' (synonyms).
'ice'+'pick'='ICE-PICK'
(Other definitions for ice pick that I've seen before include "Sharp tool" , "murder weapon?" , "Equipment used by high-level climbers" , "Mountain climber's tool" , "aid" .)