Invaders succeeded with right to enter Donne's island? (7)
I believe the answer is:
normans
'invaders' is the definition.
(Norman Conquest)
'succeeded with right to enter donne's island?' is the wordplay.
'succeeded' becomes 's' (genealogical abbreviation).
'with' is a charade indicator (letters next to each other).
'right' becomes 'r' (common abbreviation).
'to enter' means one lot of letters goes inside another.
'donne's island?' becomes 'noman' ('no man is an island' - poet John Donne).
'r' placed into 'noman' is 'norman'.
's' after 'norman' is 'NORMANS'.
(Other definitions for normans that I've seen before include "11th century invaders" , "William the Conqueror's lot?" , "They won the battle of Hastings" , "French folk" , "Some Frenchmen" .)