Something served to soldier (vegetarian) (6)
I believe the answer is:
radish
'something served' is the definition.
I can't tell whether this definition defines the answer.
'soldier vegetarian' is the wordplay.
'soldier' becomes 'RA' (member of the Royal Artillery).
'vegetarian' becomes 'dish' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ra'+'dish'='RADISH'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for radish that I've seen before include "Pungent, red salad vegetable" , "Hard vegetable" , "Edible small root" , "Root vegetable used in salads" , "Daikon or mooli" .)