Crossword Genius

In the legal position adopted by Edison (6)

Ross

I believe the answer is:

onside

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'in' is the definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?

'legal position adopted by edison' is the wordplay.
'edison' can be anagrammed to 'ONSIDE'.
But, I'm unsure how this is indicated.

'the' is the link.

Can you help me to learn more?

(Other definitions for onside that I've seen before include "in a position to score?" , "In OK position to achieve goal" , "In position to play" , "Working towards the same goal" , "in a permitted position in sport" .)

I've seen this clue in the Evening Standard.
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