Crossword Genius

Just but obsolete law (5)

Ross

I believe the answer is:

exact

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'just' is the definition.
Both the definition and answer are adjectives. Maybe there's a link between them I don't understand?

'obsolete law' is the wordplay.
'obsolete' becomes 'ex' (I can't explain this - if you can you should believe this answer much more).
'law' becomes 'act' (I've seen this in another clue).
'ex'+'act'='EXACT'

'but' acts as a link.

Can you help me to learn more?

(Other definitions for exact that I've seen before include "Precisely accurate" , "Finished" , "accurate in all details" , "Call" , "correct in every detail" .)

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