Right to block incorrect grade in Kent school (12)
I believe the answer is:
kindergarten
'school' is the definition.
(I know that kindergarten is a type of school)
'right to block incorrect grade in kent' is the wordplay.
'right' becomes 'r' (common abbreviation).
'to block' means one lot of letters goes inside another (block can mean to plug a gap).
'incorrect' is an anagram indicator.
'grade'+'in'+'kent'='gradeinkent'
'gradeinkent' is an anagram of 'kindegarten'.
'r' going into 'kindegarten' is 'KINDERGARTEN'.
(Other definitions for kindergarten that I've seen before include "Hamburger's first class" , "Children's school" , "Pre-school class" , "Playgroup" , "Nursery school" .)