Is not wasteful with the jam (9)

'the jam' is the definition.
The definition and answer can be both things that one consumes for nourishment as well as being singular nouns.
Maybe they are linked in a way I don't understand?

'is not wasteful' is the wordplay.
'is not' becomes 'con' (I've seen this in other clues).
'wasteful' becomes 'serves' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'con'+'serves'='CONSERVES'

'with' is the link.

Can you help me to learn more?