A herb (5)
I believe the answer is:
thyme
'a herb' is the definition.
(I know that thyme is a type of Herb)
This is the entire clue.
(Other definitions for thyme that I've seen before include "Shrub - herb" , "Shrubby herb" , "Aromatic culinary herb; sounds like occasion" , "Plant of the mint family" , "Ingredient in cooking" .)