Egg-laying mammal can hide nuts (7)
I believe the answer is:
echidna
'egg-laying mammal' is the definition.
(echidna is a kind of egg-laying mammal)
'can hide nuts' is the wordplay.
'nuts' indicates an anagram (nuts can mean crazy).
'can'+'hide'='canhide'
'canhide' is an anagram of 'ECHIDNA'.
(Other definitions for echidna that I've seen before include "Creature with spines" , "This insectivore" , "Toothless type" , "Prickly creature" , "Spiny creature" .)