The most abundant element in the universe (8)
I believe the answer is:
hydrogen
'the most abundant element in the universe' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for hydrogen that I've seen before include "Element with atomic number 1" , "Lightest of all elements" , "The commonest element" , "Gas, symbol H" , "... ditto: symbol H" .)