Betraying monarch elates baron terribly (11)

I believe the answer is:

treasonable

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'betraying monarch' is the definition.
'treasonable' can be an answer for 'betraying' (thesaurus). I am not certain of the 'monarch' bit.

'elates baron terribly' is the wordplay.
'terribly' indicates an anagram.
'elates'+'baron'='elatesbaron'
'elatesbaron' with letters rearranged gives 'TREASONABLE'.

Can you help me to learn more?

(Other definitions for treasonable that I've seen before include "worthy of conviction" , "aiming to put down the boss?" , "giving people away" , "Guilty of betrayal" , "helping enemy" .)

I've seen this clue in The Times.

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