Clandestine group surrounding English king (6)
I believe the answer is:
secret
'clandestine' is the definition.
(synonyms)
'group surrounding english king' is the wordplay.
'group' becomes 'set' (both can mean a collection).
'surrounding' is an insertion indicator.
'english' becomes 'E' (abbreviation).
'king' becomes 'CR' (short for Carolus Rex, King Charles in Latin).
'e'+'cr'='ecr'
'set' going around 'ecr' is 'SECRET'.
(Other definitions for secret that I've seen before include "hushed prayer" , "Not to be disclosed" , "Kept hidden from others" , "Confidence" , "Cloak-and-dagger" .)