A fool is told off (4)
I believe the answer is:
dolt
'a fool' is the definition.
(I know that fool can be written as dolt)
'told off' is the wordplay.
'off' is an anagram indicator.
'told' with letters rearranged gives 'DOLT'.
'is' is the link.
(Other definitions for dolt that I've seen before include "Slow-witted person" , "Dullard" , "dope" , "Simpleton" , "Dunce" .)