Crossword Genius

Young dog and a smaller youngster (4)

Ross

I believe the answer is:

pupa

I'm a little stuck... Click here to teach me more about this clue!

'smaller youngster' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the 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" .)

I've seen this clue in The Mirror.
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