Is not wasteful with the jam (9)

'the jam' is the definition.
The answer and definition can be both things that one consumes for nourishment as well as being singular nouns.
Perhaps you can see a link between them that I can't see?

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

'with' is the link.

Can you help me to learn more?