I'm going for toast (7)
I believe the answer is:
cheerio
I believe this is a double definition.
'i'm going' is the first definition.
(word said when leaving)
'toast' is the second definition.
(I've seen this before)
'for' is the link.
(Other definitions for cheerio that I've seen before include "for so long!" , "See you" , "A hearty farewell, begins with applause" , "Ta-ta" , "Bye-bye" .)