Just but obsolete law (5)
I believe the answer is:
exact
'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" .)