Toast and two-thirds of Stilton, perhaps with port (7)
I believe the answer is:
cheerio
'toast' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
(Other definitions for cheerio that I've seen before include "Farewell - drinking toast" , "An informal hearty goodbye" , "Ta-ta" , "Say this before drinking" , "Expression used when leaving" .)