Don't be wasteful with the jam (8)
I believe the answer is:
conserve
'the jam' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for conserve that I've seen before include "Save (for later use)" , "Store carefully; jam" , "Save - jam with whole pieces of fruit" , "Protect from destruction" , "Converse about jam." .)