Cheers poison-swallowing sovereign (7)
I believe the answer is:
'sovereign' is the definition.
(I know that tsarina is a type of emperor)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for tsarina that I've seen before include "companion of emperor" , "Nicholas's wife" , "Catherine, say" , "queen" , "Wife of an old Russian emperor" .)