In defeat he named a Greek goddess (6)
I believe the answer is:
athena
'greek goddess' is the definition.
(Greek goddess of wisdom)
'in defeat he named a' is the wordplay.
'in' indicates a hidden word.
'ATHENA' is hidden in the letters of 'defeat he named a'.
(Other definitions for athena that I've seen before include "wise but fierce goddess" , "Greek goddess of wisdom etc." , "One-time goddess" , "Greek goddess of a capital city" .)