Two-thirds of official outing is dross (4)
I believe the answer is:
junk
This could be a double definition.
'two-thirds of official outing' is the first definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'dross' is the second definition.
The definition and answer can be both substances as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'is' is the link.
Can you help me to learn more?
(Other definitions for junk that I've seen before include "Rubbish - Chinese boat" , "Schlock" , "ship" , "Discarded items - Chinese sailing vessel" , "Transport" .)