Goddess has proven useful, in part (5)
I believe the answer is:
venus
'goddess' is the definition.
(Roman goddess of love)
'has proven useful in part' is the wordplay.
'in part' indicates the answer is hidden within the clue.
'VENUS' is hidden within 'has proven useful'.
(Other definitions for venus that I've seen before include "Planet; goddess" , "Second nearest planet to the sun" , "Next planet in" , "Roman counterpart of Aphrodite" , "Heavenly body" .)