It's a sweet Oriental vessel to an alien (6)
I believe the answer is:
junket
'it's a sweet' is the definition.
(junket is a kind of sweet)
'oriental vessel to an alien' is the wordplay.
'oriental vessel' becomes 'junk' (type of Asian ship. I am not sure about the 'oriental' bit.).
'to an alien' becomes 'ET' (extra terrestrial).
'junk'+'et'='JUNKET'
Can you help me to learn more?
(Other definitions for junket that I've seen before include "Excursion at public expense - type of dessert" , "food" , "Outing" , "Milky dessert" , "Useless official tour at public expense" .)