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