One behaving ineffectively with mischievous child (4)
I believe the answer is:
wimp
'one behaving ineffectively' is the definition.
(an ineffective or feeble person)
'with mischievous child' is the wordplay.
'with' becomes 'w' (abbreviation).
'mischievous child' becomes 'imp' (I've seen this before).
'w'+'imp'='WIMP'
(Other definitions for wimp that I've seen before include "Feeble or ineffective man" , "Puny, pathetic individual" , "Informally, an ineffective man" , "Milksop" , "Spineless man" .)