Goddess subsequently entering advanced area (6)
I believe the answer is:
athena
'goddess' is the definition.
(I've seen this before)
'subsequently entering advanced area' is the wordplay.
'subsequently' becomes 'then' (synonyms).
'entering' indicates putting letters inside.
'advanced' becomes 'a' (abbreviation).
'area' becomes 'a' (maths abbreviation).
'a'+'a'='aa'
'then' placed within 'aa' is 'ATHENA'.
(Other definitions for athena that I've seen before include "Greek goddess of a capital city" , "wise but fierce goddess" , "Greek goddess of wisdom etc." , "One-time goddess" .)