Smallest stone under meadow (5)
I believe the answer is:
least
'smallest' is the definition.
(synonyms)
'stone under meadow' is the wordplay.
'stone' becomes 'st'.
'under' is a charade indicator (letters next to each other).
'meadow' becomes 'lea' (a lea is a grassy area).
'st' after 'lea' is 'LEAST'.
(Other definitions for least that I've seen before include "Minimum" , "Lowest or smallest" , "To the lowest degree" , "Most slight" , "superlatively small" .)