Soldiers probing climber for key material (5)
I believe the answer is:
ivory
'key material' is the definition.
(I've seen this in another clue)
'soldiers probing climber' is the wordplay.
'soldiers' becomes 'OR' (Other Ranks).
'probing' means one lot of letters goes inside another (probe can mean to make a hole in).
'climber' becomes 'ivy' (ivy is a climbing plant).
'or' placed into 'ivy' is 'IVORY'.
'for' acts as a 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?" .)