Magic worker in children's competition (8,3)
I believe the answer is:
spelling bee
'competition' is the definition.
(spelling bee is a kind of competition)
'magic worker in children's' is the wordplay.
I cannot really see how this works, but
'magic' could be 'spelling' (I've seen this before) and 'spelling' is present in the answer.
'worker' could be 'bee' (a worker bee) and 'bee' is found within the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for spelling bee that I've seen before include "Competition to list the letters of a word" , "Game" , "competitive recitals?" , "Letter naming competition" , "Competition involving words" .)