Duck takes drink by the litre (4)
I believe the answer is:
teal
'duck' is the definition.
(teal is a type of duck)
'drink by the litre' is the wordplay.
'drink' becomes 'tea' (tea is a drink).
'by' means one lot of letters go next to another.
'the litre' becomes 'L' (abbreviation).
'tea'+'l'='TEAL'
'takes' acts as a link.
(Other definitions for teal that I've seen before include "Variety of duck" , "Type of duck" , "Variety of small ducks" , "bird" , "Small, short-necked duck" .)