Our earthly one is not mercurial (6)
I believe the answer is:
planet
'not mercurial' is the definition.
I can't tell whether this defines the answer.
'our earthly one' is the wordplay.
I cannot really understand how this works, but
'one' could be 'ane' (synonyms) and 'ane' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
'is' is the link.
Can you help me to learn more?
(Other definitions for planet that I've seen before include "World" , "Orbiting heavenly body" , "body in orbit" , "Sun's orbiter" , "Venus, for example" .)