Legally, as a rule (2,3)

I believe the answer is:

by law

I believe this is a double definition.

'legally' is the first definition.
('by law' can be a synonym of 'legally')

'a rule' is the second definition.
(I've seen this before)

'as' is the link.

(Other definitions for by law that I've seen before include "Local rule" , "Regulation" .)

I've seen this clue in the Evening Standard.

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