Soldiers probing climber for key material (5)
I believe the answer is:
ivory
'key material' is the definition.
(I've seen this before)
'soldiers probing climber' is the wordplay.
'soldiers' becomes 'OR' (military abbreviation for other ranks).
'probing' indicates putting letters inside (probe can mean to poke or prod into).
'climber' becomes 'ivy' (ivy is a climbing plant).
'or' put inside 'ivy' is 'IVORY'.
'for' is the link.
(Other definitions for ivory that I've seen before include "Yellowish white - material from tusks" , "Country on the Gulf of Guinea, a former French protectorate" , "Piano key" , "Creamy white" , "recluse's tower?" .)