Child starts to behave and is rarely naughty (5)
I believe the answer is:
bairn
'child' is the definition.
(Scottish and northern English term for a child)
'starts to behave and is rarely naughty' is the wordplay.
'starts to' says to take the initial letters.
The first letters of 'behave and is rarely naughty' is 'BAIRN'.
(Other definitions for bairn that I've seen before include "little one in Yorkshire" , "Child in Scotland" , "Scot. child" , "Kid" , "A Scottish child?" .)