Plain wicked, swallowing litres (4)
I believe the answer is:
bald
'plain' is the definition.
(both can mean unadorned)
'wicked swallowing litres' is the wordplay.
'wicked' becomes 'bad' (associated in meaning).
'swallowing' means one lot of letters goes inside another.
'litres' becomes 'l' (abbreviation).
'bad' going around 'l' is 'BALD'.
(Other definitions for bald that I've seen before include "Lacking hair on the scalp" , "With the tread worn away" , "Not covered with hair" , "Unadorned" , "Plain; hairless" .)