Little dog sits on a would-be moth (4)
I believe the answer is:
pupa
'would-be moth' is the definition.
The answer and definition can be both animals as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'little dog sits on a' is the wordplay.
'little dog' becomes 'pup' (short for puppy).
'sits on' means one lot of letters go next to another.
'pup'+'a'='PUPA'
Can you help me to learn more?
(Other definitions for pupa that I've seen before include "Phase of insect life" , "Immature insect" , "So peacock may have appeared" , "creepy-crawly" , "Stage of an insect's development" .)