The "E" may be wrong as you suggest, ...making "star" the obvious solution, but I also came up with "mega" (big) as in girl's name =Meg and "a" being a singular ingredient in "custard".
The clue is specifically singular.
That would of course mean I have the "R" wrong.
At present I'm leaning towards "star", but shouldn't the clue then read "ingredientS"?