Expand appointment to drink one litre (6)
I believe the answer is:
dilate
'expand' is the definition.
(thesaurus)
'appointment to drink one litre' is the wordplay.
'appointment' becomes 'date' ('date' can be a synonym of 'appointment').
'to drink' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'litre' becomes 'l' ('l' can be a synonym of 'litre').
'i'+'l'='il'
'date' enclosing 'il' is 'DILATE'.
(Other definitions for dilate that I've seen before include "Tailed (anag)" , "Enlarge - detail (anag)" , "Speak or write at length - widen" , "Become wider or more open" , "Widen, distend" .)