Element of social media site nobody backed (5)
I believe the answer is:
xenon
'element' is the definition.
(I know that xenon is a type of element)
'social media site nobody backed' is the wordplay.
'social media site' becomes 'X' (social media site formerly known as Twitter).
'nobody' becomes 'none' (not a single person).
'backed' is a reversal indicator.
'none' back-to-front is 'enon'.
'x'+'enon'='XENON'
'of' acts as a link.
(Other definitions for xenon that I've seen before include "Noble gas used in lamps" , "member of nobility?" , "Inert gas used in radio valves" , "Gas used in bright lamps" , "Inert 18" .)