He is one of those significant contributors (8)
I believe the answer is:
elements
'he is one of those significant contributors' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'he is' is the wordplay.
I cannot really understand how this works, but
'he' could be 'lem' and 'lem' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for elements that I've seen before include "First principles of the weather" , "He and I are two" , "Basic parts" , "Atmospheric forces like wind and rain" , "Carbon, iron and 8 Across, say" .)