A growing boy in a car! (5)
I believe the answer is:
cedar
'a growing' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'boy in a car' is the wordplay.
'boy' becomes 'Ed' (short for Edward).
'in' is an insertion indicator.
'ed' put within 'car' is 'CEDAR'.
Can you help me to learn more?
(Other definitions for cedar that I've seen before include "representative of Lebanon?" , "Evergreen tree" , "Tall coniferous tree" , "Great hardwood tree associated with Lebanon" , "Raced around noble tree" .)