A star drinks litre in mineral water (11)
I believe the answer is:
apollinaris
'mineral water' is the definition.
(I've seen this before)
'a star drinks litre in' is the wordplay.
'star' becomes 'polaris' (star within the constellation Ursa Minor).
'drinks' indicates putting letters inside (inserted letters are consumed or drunk).
'litre' becomes 'L' (abbreviation).
'l'+'in'='lin'
'polaris' enclosing 'lin' is 'pollinaris'.
'a'+'pollinaris'='APOLLINARIS'
(Other definitions for apollinaris that I've seen before include "mineral water" , "Old heretic" .)