Poor toddler who ate last of food must be asleep (4,2,3,5)
I believe the answer is:
dead to the world
'asleep' is the definition.
(thesaurus)
'poor toddler who ate last of food' is the wordplay.
'poor' is an anagram indicator.
'last of' suggests the final letters.
The final letter of 'food' is 'd'.
'toddler'+'who'+'ate'='toddlerwhoate'
'toddlerwhoate' with letters rearranged gives 'deadtotheworl'.
'deadtotheworl'+'d'='DEAD TO THE WORLD'
'must be' acts as a link.
(Other definitions for dead to the world that I've seen before include "Sleeping" , "Fast asleep" , "Sound asleep; unconscious" .)