Herb in pesto sauce (5)
I believe the answer is:
basil
'herb in pesto sauce' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for basil that I've seen before include "Herb (from Fawlty Towers?)" , "serially chaotic character" , "A herb; --- Brush, Fawlty" , "Aromatic leaves, used fresh or dried" , "Herb - boy's name" .)