Every single bit of knowledge a child has (4)
I believe the answer is:
each
'every single' is the definition.
(I've seen this in another clue)
'bit of knowledge a child has' is the wordplay.
'bit of' says the answer is hidden in the clue.
'EACH' is hidden in the letters of 'knowledge a child has'.
(Other definitions for each that I've seen before include "Per person" , "Everyone separately" , "Every single one, person or thing" , "Singly, per capita" , "...per head" .)