Moon best seen orbiting second of planets by day (9)
I believe the answer is:
satellite
'moon' is the definition.
(thesaurus)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for satellite that I've seen before include "outpost" , "Space traveller" , "Subordinate" , "Moon" , "Celestial body orbiting around another" .)