Promise to refrain from drinking toast (6)
I believe the answer is:
pledge
This clue might be a double definition.
'promise' is the first definition.
(pledging is a kind of promising)
'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' is the link.
Can you help me to learn more?
(Other definitions for pledge that I've seen before include "Vow, affirm" , "Firm commitment" , "Covenant" , "Deposit" , "Bond" .)