Spy with another protecting English royal line (11)

I believe the answer is:

plantagenet

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'royal line' is the definition.
(I know that Plantagenet is a type of royal line)

'spy with another protecting english' is the wordplay.
'spy' becomes 'plant' (someone planted somewhere to spy).
'with' means one lot of letters goes inside another.
'another' becomes 'e' (I've seen this in other clues).
'protecting english' becomes 'agent' (I can't justify this - if you can you should give a lot more credence to this answer).
'e' placed inside 'agent' is 'agenet'.
'plant'+'agenet'='PLANTAGENET'

Can you help me to learn more?

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

I've seen this clue in The Independent.

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