Basil-based sauce (5)
I believe the answer is:
pesto
'basil-based sauce' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for pesto that I've seen before include "Italian sauce containing basil" , "Green sauce" , "Ligurian food" , "Pasta sauce based on basil" , "Sauce of basil, pine nuts, etc" .)