Varnishes Otto's agreed to put on slates (6)
I believe the answer is:
japans
'varnishes' is the definition.
(I've seen this in another clue)
'otto's agreed to put on slates' is the wordplay.
'otto's agreed' becomes 'ja' (I can't explain this - if you can you should believe this answer much more).
'to put on' says to put letters next to each other.
'slates' becomes 'pans' (both can mean criticises).
'ja'+'pans'='JAPANS'
Can you help me to learn more?
(Another definition for japans that I've seen is " Varnishes".)