Clue: Element divider flexibly accommodating ununoctium (the last) (6,9)
Answer: DMITRI MENDELEEV
Explanation: Element divider flexibly accommodating ununoctium (the last) (6,9)
An envelope ('accommodating') of M ('ununoctiu**M** (the last)') in DMITRIENDELEEV, an anagram ('flexibly') of 'element divider'. Mendeleev developed the Periodic Table of elements; ununoctium is currently the last known element in the table, so the clue has an extended definition.