Savoury jelly originally served in a popular film (5)
I believe the answer is:
aspic
'savoury jelly' is the definition.
(I've seen this before)
'originally served in a popular film' is the wordplay.
'originally' says to take the initial letters.
'in' indicates putting letters inside.
'popular film' becomes 'pic' (short for picture).
The initial letter of 'served' is 's'.
'a'+'pic'='apic'
's' put within 'apic' is 'ASPIC'.
(Other definitions for aspic that I've seen before include "Jelly based on fish or meat stock" , "Savoury jelly for coating meat, eggs etc" , "Savoury jelly for coating meats and eggs" , "garnish" , "Savoury jelly used to coat eggs etc" .)