Embed spy to guard European royal family (11)
I believe the answer is:
plantagenet
'royal family' is the definition.
(I've seen this before)
'embed spy to guard european' is the wordplay.
'embed' becomes 'plant' (synonyms).
'spy' becomes 'agent' (secret agent).
'to guard' is an insertion indicator.
'european' becomes 'E' (abbreviation e.g. EU).
'agent' placed around 'e' is 'agenet'.
'plant'+'agenet'='PLANTAGENET'
(Other definitions for plantagenet that I've seen before include "royal dynasty" , "9 [HOUSE]" , "Member of English royal house from Henry II to Richard III" , "Former English royal family" .)