Pesto base (5)
I believe the answer is:
basil
'pesto base' 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 "A herb; Cardinal --- Hume" , "Herb - Mr Fawlty" , "Herb (from Fawlty Towers?)" , "Bails out the herb" , "Boy - herb" .)