Frivolous trip made at public expense (6)
I believe the answer is:
junket
(Other definitions for junket that I've seen before include "Useless official tour at public expense" , "Journey taken for pleasure (paid for at public expense?)" , "Extravagant trip by an official at public expense" , "A dessert and a trip taken at public expense" , "Lavish trip at public expense; milk dish" .)