Partisan established around island (6)
I believe the answer is:
biased
'partisan' is the definition.
(with a bias for one side or party)
'established around island' is the wordplay.
I cannot quite see how this works, but
'island' could be 'i' (geographical abbreviation) and 'i' is found in the answer.
'established' could be 'based' ('base' can be a synonym of 'establish') and 'based' is found within the remaining letters.
No letters remain.
This explanation may well be incorrect...
(Other definitions for biased that I've seen before include "Prejudiced or partisan" , "Favouring one person or side over another" , "not fair" , "favouring a particular idea" , "'One-sided, slanted (6)'" .)