Earth's circle lacks nothing (6)
I believe the answer is:
ground
'earth's' is the definition.
(synonyms)
'circle lacks nothing' is the wordplay.
I cannot quite understand how this works, but
'circle' could be 'round' (synonyms) and 'round' is found in the answer.
A single letter 'g' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for ground that I've seen before include "Soil - don't allow to fly - reason" , "Stop (a pilot) flying" , "foundation course?" , "Pulverised; land" , "Minced" .)