Radioactive element? Before noon, Curie mixed around one million (9)
I believe the answer is:
americium
'radioactive element?' is the definition.
(I've seen this before)
'before noon curie mixed around one million' is the wordplay.
'before noon' becomes 'AM' (as in AM on a clock).
'mixed' is an anagram indicator.
'around' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'million' becomes 'm' (abbreviation).
'curie' is an anagram of 'ericu'.
'ericu' placed around 'i' is 'ericiu'.
'am'+'ericiu'+'m'='AMERICIUM'
(Other definitions for americium that I've seen before include "Radioactive element" , "Emits gamma radiation" .)