Small bottle (4)
'small bottle' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for vial that I've seen before include "Bottle that contains a drug" , "Small bottle used for medicines, laboratory work, etc" , "Small glass container" , "Not much bottle" , "Small medicine bottle" .)
(Other definitions for cruet that I've seen before include "Small container for seasoning or relish at table" , "Truce (anag)" , "Set of containers for salt, pepper, oil, vinegar etc" , "Vinegar bottle" , "Small container for salt or pepper" .)