This is radioactive waste not beginning to eat a metal (8)
I believe the answer is:
astatine
'this is radioactive' is the definition.
(I know that astatine is radioactive)
'waste not beginning to eat a metal' is the wordplay.
'not beginning' means to remove the first letter.
'to eat' indicates putting letters inside.
'metal' becomes 'tin' (tin is a kind of metal).
'waste' with its first letter taken away is 'aste'.
'a'+'tin'='atin'
'aste' enclosing 'atin' is 'ASTATINE'.
(Other definitions for astatine that I've seen before include "radioactive material" , "Chemical element At" , "Radioactive element" , "Atomic number 85" , "Rare element" .)