An unruly one might be told off and left inside (5)
I believe the answer is:
child
'an unruly one might' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'told off and left inside' is the wordplay.
I cannot really see how this works, but
'left' could be 'l' (common abbreviation) and 'l' is found within the answer.
'told off' could be 'chid' (I've seen this before) and 'chid' is found in the remaining letters.
No letters remain.
This explanation may well be incorrect...
'be' acts as a link.
Can you help me to learn more?
(Other definitions for child that I've seen before include "Minor" , "Little one" , "Juvenile" , "Son or daughter" , "Toddler" .)