Assumption, not new, upset an element (9)
I believe the answer is:
potassium
'an element' is the definition.
(potassium is a kind of element)
'assumption not new upset' is the wordplay.
'not' is a deletion indicator.
'new' becomes 'n'.
'upset' is an anagram indicator.
'assumption' with 'n' removed is 'assumptio'.
'assumptio' with letters rearranged gives 'POTASSIUM'.
(Other definitions for potassium that I've seen before include "Silvery metallic element" , "Soft reactive metal" , "Soft metal, symbol K" , "Soft metallic element" , "Element K" .)