Metal in fact inert, partly (3)
I believe the answer is:
tin
'metal' is the definition.
(I know that tin is a type of metal)
'in fact inert partly' is the wordplay.
'in' says the answer is hidden in the clue.
'TIN' can be found hidden inside 'fact inert partly'.
(Other definitions for tin that I've seen before include "With copper, makes bronze" , "Ready" , "Silvery white metal" , "Money" , "Kind of metal often used as food container" .)