Young dog and a smaller youngster (4)
I believe the answer is:
pupa
'smaller youngster' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'young dog and a' is the wordplay.
'young dog' becomes 'pup' (short for puppy).
'and' says to put letters next to each other.
'pup'+'a'='PUPA'
Can you help me to learn more?
(Other definitions for pupa that I've seen before include "not yet adult" , "Immature insect" , "developing creature" , "Phase of insect life" , "Developing butterfly" .)