Chapter on mischievous child with ape (5)
I believe the answer is:
chimp
'ape' is the definition.
(chimp is a kind of ape)
'chapter on mischievous child' is the wordplay.
'chapter' becomes 'ch'.
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'mischievous child' becomes 'imp' (imp can mean a misbehaving child).
'ch'+'imp'='CHIMP'
'with' acts as a link.
(Other definitions for chimp that I've seen before include "Creature" , "A great ape (abbr.)" , "Ape (colloq.)" , "Anthropoid ape" , "Cheeky primate" .)