Herb, a wise guy! (4)
I believe the answer is:
sage
This clue is a double definition.
'herb' is the first definition.
(I know that sage is a type of herb)
'a wise guy' is the second definition.
(I've seen this in another clue)
(Other definitions for sage that I've seen before include "cheesemongers associate with Derby" , "Herb for the wise one" , "A herb or a guru" , "philosopher" , "___ and onion" .)