Solemn promise to refrain from drinking toast (6)
I believe the answer is:
Here is my best explanation:
This clue could be a double definition.
'solemn promise' is the first definition.
(I've seen this before)
'refrain from drinking toast' is the second definition.
'pledge' can be an answer for 'toast' (synonyms). I am unsure of the 'refrain from drinking' bit.
'to' acts as a link.