Lawfully through certain sides (2,6)

I believe the answer is:

by rights

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'lawfully' is the definition.
Both the answer and definition are adverbs. Perhaps there's an association between them I don't understand?

'through certain sides' is the wordplay.
'through' becomes 'by'.
'certain sides' becomes 'rights' (I am not sure about this - if you are sure you should believe this answer much more).
'by'+'rights'='BY RIGHTS'

Can you help me to learn more?

(Other definitions for by rights that I've seen before include "If things were fair" , "Legitimately" .)

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

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