It's opposed to cleaner receiving casual welcome (8)
I believe the answer is:
filthier
'it's opposed to cleaner receiving casual' is the definition.
I can't tell whether this defines the answer.
'welcome' is the wordplay.
I cannot really understand how this works, but
'welcome' could be 'hi' (both are greetings) and 'hi' is present in the answer.
The remaining letters 'filter' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Another definition for filthier that I've seen is " Disgustingly dirtier".)