Don Juan's undone and missing a godly partner (4)
I believe the answer is:
juno
'godly partner' is the definition.
(wife of Jupiter in Roman mythology)
I can't explain the remainder of the clue.
(Other definitions for juno that I've seen before include "Jupiter's wife" , "In Roman myth, goddess ofmarriage - to Paycock?" , "Roman queen of the gods, wife and sister of Jupiter" , "She played with the Paycock" , ". . . . and the Paycock (Sean O'Casey)" .)