Herb used in pesto (5)
I believe the answer is:
basil
'herb used in pesto' 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 "Culinary plant" , "Herb - Mr Fawlty" , "named Cardinal Hume" , "Sweet-smelling herb" , "Herb of mint family" .)