Herb is a wise old fellow (4)
I believe the answer is:
sage
'herb' is the definition.
(I know that sage is a type of herb)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for sage that I've seen before include "Savant" , "Fragrant herb or wise man" , "Profoundly wise" , "Wise woman" , "Fragrant herb having wisdom" .)