Solemn promise to refrain from drinking toast (6)
I believe the answer is:
pledge
This clue might 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 not certain of the 'refrain from drinking' bit.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for pledge that I've seen before include "Promise - security" , "make toast" , "Firm commitment" , "Vow; security" , "Promise (to abstain?)" .)