Going through litre bottle (4)
I believe the answer is:
vial
'bottle' is the definition.
(vial is a kind of bottle)
'going through litre' is the wordplay.
'going through' becomes 'via' (I've seen this before).
'litre' becomes 'l' ('l' can be a synonym of 'litre').
'via'+'l'='VIAL'
(Other definitions for vial that I've seen before include "Small bottle used for medicines, laboratory work, etc" , "Not much bottle" , "Bottle that contains a drug" , "Small medicine bottle" , "Small glass container" .)