Tartar sauce ingredient (4)
I believe the answer is:
mayo
'tartar sauce ingredient' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for mayo that I've seen before include "County of Western Eire" , "Irish county; dressing" , "Eg, Braeburn or Cox" , "Salad dressing (abbr)" , "You'll find it in the Emerald Isle" .)