Mine uranium in site for processing radioactive element (11)
I believe the answer is:
einsteinium
'radioactive element' is the definition.
'mine uranium in site for processing' is the wordplay.
'uranium' becomes 'U' (U is the chemical symbol for uranium).
'for processing' is an anagram indicator.
'mine'+'u'+'in'+'site'='mineuinsite'
'mineuinsite' anagrammed gives 'EINSTEINIUM'.
(Other definitions for einsteinium that I've seen before include "Synthetic element" , "Radioactive metal" , "item on table" , "Metallic element named after famous scientist" .)