What toddler does — briefly fidgeting when fast asleep (4,2,3,5)
I believe the answer is:
dead to the world
'fast asleep' is the definition.
(dead to the world can mean asleep)
'what toddler does briefly fidgeting' is the wordplay.
'briefly' means to remove the last letter.
'fidgeting' indicates an anagram (letters fidget or move around).
'what'+'toddler'+'does'='whattoddlerdoes'
'whattoddlerdoes' with its last letter taken off is 'whattoddlerdoe'.
'whattoddlerdoe' anagrammed gives 'DEAD TO THE WORLD'.
'when' acts as a link.
(Other definitions for dead to the world that I've seen before include "Sleeping" , "Fast asleep" , "Sound asleep; unconscious" .)