Drink and 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' indicates putting letters inside.
'round' becomes 'o' (round can mean a round object).
'tin' becomes 'can' (synonyms).
'o'+'can'='ocan'
'e' put inside 'ocan' is 'OCEAN'.
'and' is the link.
(Other definitions for ocean that I've seen before include "A mighty sea" , "One of the Seven Seas" , "Big sea" , "It's pretty wet!" , "Quantity of water" .)