Setting agent (5)
'setting agent' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for gelatine that I've seen before include "Setting agent" , "It's used in jelly and glue" , "Animal-based gluey substance used in cooking, photography etc" , "Clear edible jelly" , "Substance used as glue" .)
(Other definitions for aspic that I've seen before include "Savoury jelly for coating meat, eggs etc" , "Lavender" , "Stock set with food" , "Savoury relish" , "Setter" .)