A drug conveyer with petty officer in a small vessel (7)
I believe the answer is:
ampoule
'small vessel' is the definition.
'ampoule' can be an answer for 'vessel' (ampoule is a kind of vessel). I am unsure of the 'small' bit.
'a drug conveyer with petty officer' is the wordplay.
'a drug conveyer' becomes 'amule' (I am not sure about this - if you are sure you should believe this answer much more).
'with' indicates putting letters inside.
'petty officer' becomes 'po' (naval abbreviation).
'amule' placed around 'po' is 'AMPOULE'.
'in a' acts as a link.
Can you help me to learn more?
(Other definitions for ampoule that I've seen before include "Sealed glass container for injection solution" , "Glass capsule" , "Capsule containing injection dose" , "Glass container for hypodermic dose" , "Small container" .)