A Japanese entertainer's part onstage is happy (6)
I believe the answer is:
geisha
'a japanese entertainer's' is the definition.
(geishas traditionally hosted or entertained men)
'part onstage is happy' is the wordplay.
'part' indicates the answer is hidden within the clue.
'GEISHA' is hidden in the letters of 'onstage is happy'.
(Other definitions for geisha that I've seen before include "Jap. entertainer" , "Female escort" , "girl from the orient" , "Trained Japanese hostess - his age (anag)" , "Japanese serving girl" .)