Mercian king hiding in walled garden (5)

I believe the answer is:

edgar

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'mercian king' is the definition.
'edgar' can be an answer for 'king' (I've seen this before). I'm unsure of the 'mercian' bit.

'hiding in walled garden' is the wordplay.
'hiding' says the answer is hidden in the clue.
'EDGAR' can be found hidden inside 'in walled garden'.

Can you help me to learn more?

(Other definitions for edgar that I've seen before include ". . . . . Allan Poe, master of the macabre" , "J. -- Hoover, FBI boss" , "King of England" , "- Rice Burroughs, US novelist" , "- Allan Poe, US writer" .)

I've seen this clue in The Times.

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