Twins finding stone in island (6)
I believe the answer is:
gemini
'twins' is the definition.
(twins are the symbol of the star sign Gemini)
'stone in island' is the wordplay.
'stone' becomes 'gem' (gemstone).
'island' becomes 'i' (geographical abbreviation).
'gem'+'in'+'i'='GEMINI'
'finding' acts as a link.
(Other definitions for gemini that I've seen before include "Twins star-sign" , "May person" , "Third zodiacal sign" , "Sign of the Twins ( 6)" , "Twins star sign" .)