Set spies to protect English dynasty (12)

I believe the answer is:

plantagenets

'dynasty' is the definition.
(I know this)

'set spies to protect english' is the wordplay.
'set' becomes 'plant' (planting is a kind of setting).
'spies' becomes 'agents' (secret agents).
'to protect' means one lot of letters goes inside another (inserted letters are protected or taken in).
'english' becomes 'E' (abbreviation as in OED).
'agents' enclosing 'e' is 'agenets'.
'plant'+'agenets'='PLANTAGENETS'

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