Soft substitute for jam (8)
I believe the answer is:
preserve
'for jam' is the definition.
(jam is a type of preserve)
'soft substitute' is the wordplay.
'soft' becomes 'p' (musical abbreviation for piano, 'soft').
'substitute' becomes 'reserve' (synonyms).
'p'+'reserve'='PRESERVE'
(Other definitions for preserve that I've seen before include "area where game is held" , "Keep from decay, corpse or food" , "Prevent from deteriorating" , "Keep - pickle - store" , "speciality" .)