A box having a good American vegetable (9)
I believe the answer is:
asparagus
'vegetable' is the definition.
(asparagus is a kind of vegetable)
'a box having a good american' is the wordplay.
'box' becomes 'spar' (sparring is a kind of boxing).
'having' says to put letters next to each other.
'good american' becomes 'gus' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a'+'spar'+'a'+'gus'='ASPARAGUS'
Can you help me to learn more?
(Other definitions for asparagus that I've seen before include "Spears or shoots" , "Food" , "Vegetable delicacy" , "Plant whose young shoots are cooked as expensive vegeable" , "Speared vegetable" .)