Cold butter and jam (4)
I believe the answer is:
cram
'jam' is the definition.
(I know that jam can be written as cram)
'cold butter' is the wordplay.
'cold' becomes 'c' (eg on taps).
'butter' becomes 'ram' (I've seen this in another clue).
'c'+'ram'='CRAM'
'and' is the link.
(Other definitions for cram that I've seen before include "Study intensively" , "Revise hard" , "Study hard for exam" , "jam" , "Stuff full" .)