Herb is a growing boy (5)
I believe the answer is:
basil
This clue could be a double definition.
'herb' is the first definition.
(I know that basil is a type of herb)
'growing boy' is the second definition.
'basil' can be an answer for 'boy' (I've seen this before). I am unsure of the 'growing' bit.
'is a' acts as a link.
Can you help me to learn more?
(Other definitions for basil that I've seen before include "Herb - Mr Fawlty" , "A herb; --- Brush, Fawlty" , "Herb of mint family" , "Aromatic herb in pesto" , "Herb; forename" .)