Herb associated with Don in Essex town (5)
I believe the answer is:
basil
I believe this is a double definition.
'herb' is the first definition.
(basil is a herb)
'associated with don in essex town' is the second definition.
(Basildon is the name of a town in Essex)
(Other definitions for basil that I've seen before include "A herb; --- Brush, Fawlty" , "A herb; Cardinal --- Hume" , "Herb; forename" , "Mediterranean herb" , "Aromatic leaves, used fresh or dried" .)