Post spy outside English royal house (11)

I believe the answer is:

plantagenet

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'royal house' is the definition.
(I've seen this before)

I don't understand how the remainder of the clue works.

(Other definitions for plantagenet that I've seen before include "Former English royal family" , "royal dynasty" , "Member of English royal house from Henry II to Richard III" , "9 [HOUSE]" .)

I've seen this clue in The Times.

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