Poor toddler who ate lizard's tail is unconscious (4,2,3,5)
I believe the answer is:
dead to the world
'unconscious' is the definition.
(thesaurus)
'poor toddler who ate lizard's tail' is the wordplay.
'poor' is an anagram indicator.
'tail' says to take the final letters.
The final letter of 'lizard' is 'd'.
'toddler'+'who'+'ate'='toddlerwhoate'
'toddlerwhoate' is an anagram of 'deadtotheworl'.
'deadtotheworl'+'d'='DEAD TO THE WORLD'
'is' is the link.
(Other definitions for dead to the world that I've seen before include "Fast asleep" , "Sleeping" , "Sound asleep; unconscious" .)