And drink up about a litre (4)
I believe the answer is:
plus
'and' is the definition.
'drink up about a litre' is the wordplay.
'drink' becomes 'sup' (to sup is to drink).
'up' is a reversal indicator (in down clue: letters go upwards).
'about' is an insertion indicator.
'a litre' becomes 'L' (abbreviation).
'sup' backwards is 'pus'.
'pus' placed around 'l' is 'PLUS'.
(Other definitions for plus that I've seen before include "asset" , "Together with; advantage" , "The opposite of minus" , "Something more" , "Coupled with" .)