Create atmosphere partly by use of onyx, for example (6)
I believe the answer is:
oxygen
'example' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for oxygen that I've seen before include "Gas forming about 20 per cent of atmosphere" , "Gas that combines with hydrogen to form water" , "Atmospheric gas" , "Gas given off in photosynthesis" , "Life supporting element" .)