Egg-laying mammal
I believe the answer is:
echidna
'egg-laying mammal' is the definition.
(echidna is a kind of egg-laying mammal)
This is all the clue.
(Other definitions for echidna that I've seen before include "Insectivorous mammal" , "Australian creature" , "Little animal" , "Spiny creature" , "Spiny egg-laying Australian mammal" .)