Vessel containing cut grass that's used for soup! (6)
I believe the answer is:
tureen
'that's used for soup' is the definition.
(soup may be served from a tureen)
'vessel containing cut grass' is the wordplay.
'vessel' becomes 'tun' (type of vessel for beer).
'containing' means one lot of letters goes inside another.
'cut grass' becomes 'ree' (I can't justify this - if you can you should give a lot more credence to this answer).
'tun' enclosing 'ree' is 'TUREEN'.
Can you help me to learn more?
(Other definitions for tureen that I've seen before include "A deep covered dish for serving soup" , "A large covered dish for serving soup" , "Soup-dish" , "Large, deep serving dish with a cover" , "A large deep soup dish" .)