Endless humility formed an element (7)
I believe the answer is:
lithium
'an element' is the definition.
(lithium is a kind of element)
'endless humility formed' is the wordplay.
'endless' means to remove the last letter.
'formed' indicates an anagram.
'humility' with its final letter removed is 'humilit'.
'humilit' is an anagram of 'LITHIUM'.
(Other definitions for lithium that I've seen before include "Lightest alkali metal" , "Light metal element" , "Lightest-known metal" , "Soft silvery element" , "Chemical element (Li), used in batteries" .)