Goddess is one's double (4)
I believe the answer is:
isis
'goddess' is the definition.
(Egyptian god)
'one's double' is the wordplay.
'one's' becomes 'I's' (Roman numeral).
'double' shows that letters should be duplicated.
'is' doubled is 'ISIS'.
'is' is the link.
(Other definitions for isis that I've seen before include "Oxford river; goddess" , "Jihadists" , "Oxford's second crew" , "To be, to be an Egyptian goodess" , "banker in Oxford" .)