Spell 'slates' backwards (4)
I believe the answer is:
snap
'spell' is the definition.
(I know that snap is a type of spell)
'slates backwards' is the wordplay.
'slates' becomes 'pans' (both can mean criticises).
'backwards' says the letters should be written in reverse.
'pans' backwards is 'SNAP'.
(Other definitions for snap that I've seen before include "A walk in the park < to shoot" , "Ginger biscuit" , "Go to pieces < , it's said, quickly" , "Cut" , "Sudden break" .)