Respond concerning a decree in law (5)

I believe the answer is:

react

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'respond' is the definition.
(to react to something is to respond to it)

'a decree in law' is the wordplay.
'a decree in' becomes 're' (I can't explain this - if you can you should believe this answer much more).
'law' becomes 'act' (I've seen this before).
're'+'act'='REACT'

'concerning' is the link.

Can you help me to learn more?

(Other definitions for react that I've seen before include "Perform again" , "Trace (anag)" , "Respond to something" , "Act as opponent" , "Change chemically" .)

I've seen this clue in The Independent.

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