Child's never-ending make-believe is full of energy (3-4)

'child's' is the definition.

'never-ending make-believe is full of energy' is the subsidiary indication.
'never-ending' means to remove the last letter (the \'end\' of the word never appears).
'make believe' becomes 'pretend' (I've seen this before).
'is full of' indicates putting letters inside.
'energy' becomes 'e' (symbol in physics e.g. in Einstein's famous e=mc2).
'pretend' with its last letter taken off is 'preten'.
'preten' going around 'e' is 'preteen'.