Evil company's left an astronomer orbiting a moon (10)
I believe the answer is:
pernicious
'evil company's left an astronomer orbiting a moon' is the definition.
I can't tell whether this defines the answer.
'evil company's left an astronomer orbiting' is the wordplay.
'evil' becomes 'io' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'company' becomes 'co' (abbreviation).
'left' is a deletion indicator.
'an astronomer' becomes 'copernicus' (I've seen this before).
'orbiting' means one lot of letters goes inside another.
'copernicus' with 'co' taken away is 'pernicus'.
'io' going into 'pernicus' is 'PERNICIOUS'.
Can you help me to learn more?
(Another definition for pernicious that I've seen is " Destructive and harmful".)