Smallest sort of slate (5)
I believe the answer is:
least
'smallest' is the definition.
(synonyms)
'sort of slate' is the wordplay.
'sort of' is an anagram indicator.
'slate' with letters rearranged gives 'LEAST'.
(Other definitions for least that I've seen before include "Steal (anag)" , "Fewest" , "Smallest (in significance)" , "Minimum" , "superlatively small" .)