Occasionally seal cake in bag (4)
I believe the answer is:
sack
'bag' is the definition.
(both can mean a container)
'occasionally seal cake' is the wordplay.
'occasionally' means one should take alternating letters.
'seal'+'cake'='sealcake'
The alternating letters of 'sealcake' are 'SACK'.
'in' is the link.
(Other definitions for sack that I've seen before include "destroy > 50s dress" , "Receptacle" , "Dismiss (from a job)" , "Fire the old drink?" , "Marching orders?" .)