Hide treasure at last in small Greek island (7)
I believe the answer is:
secrete
'hide' is the definition.
(I've seen this in another clue)
'treasure at last in small greek island' is the wordplay.
'at last' suggests the final letters.
'in' is an insertion indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'greek island' becomes 'crete' (Crete is a Greek island).
The last letter of 'treasure' is 'e'.
's'+'crete'='screte'
'e' put inside 'screte' is 'SECRETE'.
(Other definitions for secrete that I've seen before include "squirrel" , "(Of gland) produce (a substance)" , "Hide or conceal an object" , "Cache" , "Place out of sight, hide" .)