Italian sauce (5)
I believe the answer is:
pesto
'italian sauce' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for pesto that I've seen before include "Pasta sauce mainly of basil and cheese" , "Sauce of basil, pine nuts and garlic" , "sauce involving Basil" , "Pasta sauce variety" , "Italian sauce containing basil" .)