Single portion fills in snack in two sections (9)
I believe the answer is:
bipartite
'single portion fills' is the definition.
Both the answer and definition are adjectives. Perhaps there's an association between them I don't understand?
'in snack in two sections' is the wordplay.
'in' indicates putting letters inside.
'snack' becomes 'bite' (synonyms).
'in two sections' becomes 'parti' (I can't justify this - if you can you should give a lot more credence to this answer).
'bite' enclosing 'parti' is 'BIPARTITE'.
Can you help me to learn more?
(Another definition for bipartite that I've seen is " Involving two sides".)