In ancient Thebes, a goddess (4)
I believe the answer is:
hebe
'a goddess' is the definition.
(I've seen this before)
'in ancient thebes' is the wordplay.
'in' says the answer is hidden in the clue.
'HEBE' can be found hidden inside 'ancient thebes'.
(Other definitions for hebe that I've seen before include "Olympian cupbearer - evergreen garden shrub" , "Cup-bearer of the gods and flowering shrub" , "Greek goddess of spring" , "Youth personified" , "Goddess of youth and flowering shrub" .)