Invader right to enter that which is claimed to be an island (6)
I believe the answer is:
Here is my best explanation:
'invader' is the definition.
'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'.