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' (Other Ranks).
'probing' is an insertion indicator (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?" .)