A growing boy with taste (5)
I believe the answer is:
basil
'a growing' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'boy with taste' is the wordplay.
'boy' becomes 'b' (abbreviation used in schools).
'with' is a charade indicator (letters next to each other).
'taste' becomes 'asil' (I can't justify this - if you can you should give a lot more credence to this answer).
'b'+'asil'='BASIL'
Can you help me to learn more?
(Other definitions for basil that I've seen before include "A herb; Cardinal --- Hume" , "Culinary herb" , "Seasoning" , "comic character" , "Sweet-smelling herb" .)