Bottle setter consumed in the flipping toilet (4)
I believe the answer is:
Here is my best explanation:
'bottle' is the definition.
(vial is a kind of bottle)
'setter consumed in the flipping toilet' is the subsidiary indication.
'setter' becomes 'i' (the person that set the crossword).
'consumed in' means one lot of letters goes inside another.
'the flipping' says the letters should be written backwards.
'toilet' becomes 'lav' ('lav' can be a synonym of 'toilet').
'lav' backwards is 'val'.
'i' placed inside 'val' is 'vial'.
(Other definitions for vial that I've seen before include "Small glass container", "Small container for liquids", "Small container for liquid medicine", "Small medicine bottle", "Medicine container", "Not much bottle", "Small bottle used for medicines, laboratory work, etc", "Medicine bottle", "small bottle" and "Bottle that contains a drug".)