It's in a bottle or other vessel with some soup (7)
I believe the answer is:
ketchup
'it's in a bottle or other' is the definition.
I can't tell whether this definition defines the answer.
'vessel with some soup' is the wordplay.
'vessel' becomes 'ketch' (ketch is a kind of vessel).
'with some soup' becomes 'up' (I am not sure about this - if you are sure you should believe this answer much more).
'ketch'+'up'='KETCHUP'
Can you help me to learn more?
(Other definitions for ketchup that I've seen before include "Sauce made largely of tomatoes" , "Spicy tomato sauce" , "Piquant sauce, often tomato based" , "Condiment" , "Relish" .)