Feeble learner, mischievous child (4)
I believe the answer is:
limp
'feeble' is the definition.
(I've seen this before)
'learner mischievous child' is the wordplay.
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'mischievous child' becomes 'imp' (I've seen this before).
'l'+'imp'='LIMP'
(Other definitions for limp that I've seen before include "Walk with an uneven step" , "Weak" , "obstacle to smooth progress" , "without energy" , "Walk with an injury" .)