Italian basil sauce (5)
I believe the answer is:
pesto
'italian basil sauce' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for pesto that I've seen before include "Pasta sauce mainly of basil and cheese" , "Basil-based sauce" , "Sauce made of basil and cheese" , "Basic sauce" , "Italian sauce containing basil" .)