South-eastern island, mostly unknown (6)
I believe the answer is:
secret
'unknown' is the definition.
(thesaurus)
'south-eastern island mostly' is the wordplay.
'south eastern' becomes 'se'.
'island' becomes 'crete' (Crete is an example).
'mostly' means to remove the last letter.
'crete' with its final letter taken off is 'cret'.
'se'+'cret'='SECRET'
(Other definitions for secret that I've seen before include "Hidden, private" , "Confidence" , "avoiding exposure" , "Inner" , "Not known by others" .)