Lager: drunk sniper drinking litre (7)
I believe the answer is:
pilsner
'lager' is the definition.
(type of lager)
'drunk sniper drinking litre' is the wordplay.
'drunk' is an anagram indicator.
'drinking' indicates putting letters inside (inserted letters are consumed or drunk).
'litre' becomes 'L' (abbreviation).
'sniper' with letters rearranged gives 'pisner'.
'pisner' going around 'l' is 'PILSNER'.
(Other definitions for pilsner that I've seen before include "Czech beer" , "Bar tab might include this" , "drink" , "Type of lager" , "alcohol" .)