Element one among former students Greek character backed (9)
I believe the answer is:
aluminium
'element' is the definition.
(I know that aluminium is a metal)
'one among former students greek character backed' is the wordplay.
'one' becomes 'i' (Roman numeral).
'among' means one lot of letters goes inside another (inserted letters are placed among others).
'former students' becomes 'alumni' (I've seen this before).
'greek character' becomes 'mu' (letter from the Greek alphabet).
'backed' says the letters should be written in reverse.
'mu' reversed gives 'um'.
'i' going inside 'alumni' is 'alumini'.
'alumini'+'um'='ALUMINIUM'
(Other definitions for aluminium that I've seen before include "Light metal" , "Lightweight metal" , "Al" , "Metallic element found in bauxite" , "Silvery metallic element" .)