Metal element (8)
(Other definitions for tantalum that I've seen before include "Acid-resistant metal" , "Metallic element, symbol Ta" .)
(Other definitions for indium that I've seen before include "Silvery-white metal" , "Metallic element, In" .)
'metal element' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for palladium that I've seen before include "Rare element" , "Misery at own condition" , "guarantee of protection" , "London variety theatre - element - safeguard" , "Theatre in London; rare metal" .)