Crossword Genius

Well across foe, ruse B? (4,9)

Ross

I believe the answer is:

know backwards

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'well' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.

'across foe ruse b?' is the wordplay.
I cannot quite understand how this works, but
'across' could be 'ac' (I've seen this in other clues) and 'ac' is found within the answer.
'b?' could be 'b' and 'b' is present in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

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