Twin of Artemis (6)
I believe the answer is:
apollo
'twin of artemis' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for apollo that I've seen before include "US space flight programme, 1961-72" , "Greek god of the sun (6)" , "Greek god (twin brother of Artemis)" , "London theatre" , "Greek god of light, poetry, music, healing and prophecy" .)