One's twice a goddess! (4)
I believe the answer is:
isis
'a goddess' is the definition.
(thesaurus)
'one's twice' is the wordplay.
'one's' becomes 'is' (Roman numeral).
'twice' says to double the letters.
'is' doubled is 'ISIS'.
(Other definitions for isis that I've seen before include "Oxford's second crew" , "Oxfordshire river" , "Egyptian deity" , "Si, si - an Egyptian goddess" , "Egyptian fertility goddess, mother of Horus" .)