Trattoria topping (5)
I believe the answer is:
pesto
'trattoria topping' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for pesto that I've seen before include "Sauce of basil, pine nuts and garlic" , "Saucy work" , "Ligurian food" , "Pasta sauce variety" , "Pasta sauce based on basil" .)