Invader right to enter that which is claimed to be an island (6)
I believe the answer is:
norman
'invader' is the definition.
(norman conquest)
'right to enter that which is claimed to be an island' is the wordplay.
'right' becomes 'r' (common abbreviation).
'to enter' indicates putting letters inside.
'that which is claimed to be an island' becomes 'noman' ('no man is an island' - John Donne).
'r' inserted inside 'noman' is 'NORMAN'.
(Other definitions for norman that I've seen before include "Boy's name" , "Person from France" , "Sir - - Hartnell" , "Ten sixty six invader" , "-- Wisdom, comedian" .)