Duck cleans litre out of seas (6)
I believe the answer is:
oceans
'seas' is the definition.
('ocean' can be a synonym of 'sea')
'duck cleans litre out' is the wordplay.
'duck' becomes 'o' (resembles zero - 'duck' in cricket).
'litre' becomes 'L' (abbreviation).
'out' indicates named letters should be taken away.
'cleans' with 'l' taken out is 'ceans'.
'o'+'ceans'='OCEANS'
'of' acts as a link.
(Other definitions for oceans that I've seen before include "Bodies of salt water" , "expanses?" , "Strange canoes on these waters" , "Arctic and Indian" , "big seas" .)