Solemn undertaking to abstain from drinking a toast (6)
I believe the answer is:
pledge
This clue is a double definition.
'solemn undertaking to abstain' is the first definition.
'drinking a toast' is the second definition.
'from' is the link.
(Other definitions for pledge that I've seen before include "Promise, word of honour" , "Promise solemnly and formally" , "Solemn undertaking" , "Promise or pawn" , "Plight" .)