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