Impiously go back of beyond minus long fork (9)

I believe the answer is:

godlessly

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'impiously' is the definition.
Both the definition and answer are adverbs.
Perhaps they are linked in a way I don't understand?

'go back of beyond minus long fork' is the wordplay.
'back of' suggests the final letters.
'minus' becomes 'less' (I've seen this before).
'long fork' becomes 'ly' (I can't justify this - if you can you should believe this answer much more).
The last letter of 'beyond' is 'd'.
'go'+'d'+'less'+'ly'='GODLESSLY'

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I've seen this clue in the Sydney Morning Herald.

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