"I'll drink to that!" (4)
I believe the answer is:
amen
'i'll drink to that' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for amen that I've seen before include "Word of agreement" , "Prayer ending" , "Conclusion of prayer" , "Word of approval" , "Word at the end of a prayer" .)