Unattached girl’s mountainous island (8)
I believe the answer is:
discrete
'unattached' is the definition.
(I've seen this before)
'girl's mountainous island' is the wordplay.
'girl' becomes 'dis' (I've seen this in other clues).
'mountainous island' becomes 'Crete' (Crete is a Greek island).
'dis'+'crete'='DISCRETE'
(Other definitions for discrete that I've seen before include "Individually self-contained in discreet way" , "independent" , "Individual" , "Separated" , "Secret ID (anag) - separate" .)