BLT topping (4)
I believe the answer is:
mayo
'blt topping' 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 "Irish county - sauce (abbr)" , "Irish county; dressing" , "You'll find it in the Emerald Isle" , "Saucy thing" , "Irish county (county town, Castlebar)" .)