Scientist finding sodium half an element (11)
I believe the answer is:
einsteinium
'an element' is the definition.
(I know that einsteinium is a synthetic element)
'scientist finding sodium half' is the wordplay.
'scientist' becomes 'einstein' (Einstein is an example).
'finding' says to put letters next to each other.
'half' indicates taking half.
'sodium' halved is 'ium'.
'einstein'+'ium'='EINSTEINIUM'
(Other definitions for einsteinium that I've seen before include "Metallic element named after famous scientist" , "Radioactive metal" , "Synthetic element" , "item on table" .)