One holds the jewel of the twins (6)
I believe the answer is:
gemini
'the twins' is the definition.
(twins are the symbol of the star sign Gemini)
'one holds the jewel' is the wordplay.
I cannot quite see how this works, but
'one' could be 'i' (Roman numeral) and 'i' is present in the answer.
'jewel' could be 'gem' (synonyms) and 'gem' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
'of' is the link.
Can you help me to learn more?
(Other definitions for gemini that I've seen before include "Sign of the Twins ( 6)" , "Castor and Pollux constellation" , "Third zodiacal sign" , "May person" , "Twins star-sign" .)