Netherlander’s Cockney wife on island (8)
I believe the answer is:
dutchman
'netherlander's' is the definition.
(someone from the Netherlands)
'cockney wife on island' is the wordplay.
'cockney wife' becomes 'Dutch' (historic Cockney slang term for wife).
'on' says to put letters next to each other.
'island' becomes 'man' (the Isle of Man is an example).
'dutch'+'man'='DUTCHMAN'
(Other definitions for dutchman that I've seen before include "European" , "Wagner composed 'The Flying ........'" , "bolster his wife" , "Native of Amsterdam, for example" , "Accursed Wagner character" .)