Holy Island could be finer island (11)
I believe the answer is:
lindisfarne
'holy island' is the definition.
(I've seen this before)
'could be finer island' is the wordplay.
'could be' indicates an anagram.
'finer'+'island'='finerisland'
'finerisland' anagrammed gives 'LINDISFARNE'.
(Other definitions for lindisfarne that I've seen before include "peninsula?" , "north-eastern island" , "Holy Island" .)