Pesto herb (5)
I believe the answer is:
basil
'pesto herb' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for basil that I've seen before include "Aromatic herb in pesto" , "Herb of mint family" , "Fragrant herb" , "Aromatic plant used as a herb" , "Herb - boy's name" .)