Kid scolded once, having crossed line (5)
I believe the answer is:
child
'kid' is the definition.
('child' can be a synonym of 'kid')
'scolded once having crossed line' is the wordplay.
'scolded once' becomes 'chid' (archaic past-tense version of 'chide').
'having crossed' indicates putting letters inside.
'line' becomes 'l'.
'chid' going around 'l' is 'CHILD'.
(Other definitions for child that I've seen before include "Young person" , "Infant" , "Little one" , "Immature human" , "Immature, irresponsible person" .)