Metal instrument, one probing stomach (8)
I believe the answer is:
lutetium
'metal' is the definition.
(lutetium is a kind of metal)
'instrument one probing stomach' is the wordplay.
'instrument' becomes 'lute' (lute is a kind of instrument).
'one' becomes 'i' (Roman numeral).
'probing' is an insertion indicator (probe can mean to make a hole in).
'stomach' becomes 'tum' ('tum' can be a synonym of 'stomach').
'i' inserted into 'tum' is 'tium'.
'lute'+'tium'='LUTETIUM'
(Other definitions for lutetium that I've seen before include "Metallic element" , "Metal" .)