Setting agent (7)
(Other definitions for gelatin that I've seen before include "Colourless protein - edible jelly" , "Colourless glue" , "Animal protein used in foods, glues, emulsions, etc" , "Substance used in glue and cooking" , "Food/glue jelly" .)
'setting agent' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for gelatine that I've seen before include "Clear substance used in jelly" , "Animal-based gluey substance used in cooking, photography etc" , "Setting agent" , "Sticky substance obtained from boiled animal tissue" , "Form of glue used in foods" .)
(Other definitions for aspic that I've seen before include "stock production" , "Ending of Marlowe's thriller" , "Jelly made with fish or meat stock" , "Jelly based on fish or meat stock" , "Savoury relish" .)