Offer the jam beforehand? (8)
I believe the answer is:
preserve
'jam beforehand?' is the definition.
'preserve' can be an answer for 'jam' (jam is a type of preserve). I am unsure of the 'beforehand?' bit.
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for preserve that I've seen before include "Maintain in unaltered condition" , "Keep alive, maintain" , "area where game is held" , "Jam or marmalade" , "Keep from decay in perverse way" .)