A smuggler hides cut drug in small glass vessel (7)
I believe the answer is:
ampoule
'small glass vessel' is the definition.
(type of glass vessel)
'a smuggler hides cut drug' is the wordplay.
'smuggler' becomes 'mule' (as in a drugs mule).
'hides' indicates putting letters inside.
'cut drug' becomes 'po' (I can't justify this - if you can you should believe this answer much more).
'mule' placed around 'po' is 'mpoule'.
'a'+'mpoule'='AMPOULE'
'in' is the link.
(Other definitions for ampoule that I've seen before include "Sealed glass container for injection solution" , "Glass container for hypodermic dose" , "Vessel holding liquid for injection" , "Glass capsule" , "Capsule containing injection dose" .)