Supply unopened jam (7)
I believe the answer is:
reserve
'supply' is the definition.
(thesaurus)
'unopened jam' is the wordplay.
'unopened' means to remove the first letter (without opening letter).
'jam' becomes 'preserve' (I've seen this before).
'preserve' with its first letter taken away is 'RESERVE'.
(Other definitions for reserve that I've seen before include "not in the front line" , "left on the bench?" , "put to one side" , "keep for oneself" , "Order" .)