Aconite, it’s dropped slyly in the drink (5)
I believe the answer is:
ocean
'the drink' is the definition.
('the drink' is an informal term for the sea)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for ocean that I've seen before include "eg Indian, Atlantic" , "Pacific, perhaps" , "Indian perhaps" , "Quantity of water" , "Indian or Arctic, say" .)