Sign of stone in island (6)
I believe the answer is:
gemini
'sign' is the definition.
(Gemini is a star sign)
'stone in island' is the wordplay.
'stone' becomes 'gem' (a gemstone).
'island' becomes 'i' (geographical abbreviation).
'gem'+'in'+'i'='GEMINI'
'of' is the link.
(Other definitions for gemini that I've seen before include "space programme" , "star group" , "set of twinklers" , "Heavenly sign of the Twins" , "Zodiacal twins" .)