Drink drug in round tin (5)
I believe the answer is:
ocean
'drink' is the definition.
('the drink' is an informal term for the sea)
'drug in round tin' is the wordplay.
'drug' becomes 'e' (e is another name for ecstasy).
'in' is an insertion indicator.
'round' becomes 'o' (round can mean a round object**).
'tin' becomes 'can' ('can' can be a synonym of 'tin').
'o'+'can'='ocan'
'e' inserted inside 'ocan' is 'OCEAN'.
(Other definitions for ocean that I've seen before include "Main" , "It's pretty wet!" , "Huge expanse" , "tons of water" , "Indian perhaps" .)