Small mature herb (4)
I believe the answer is:
sage
'herb' is the definition.
(I know that sage is a type of herb)
'small mature' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'mature' becomes 'age' (synonyms).
's'+'age'='SAGE'
(Other definitions for sage that I've seen before include "Herb; wise person" , "Herb for the wise one" , "Seasoning for the wise one" , "Solon, for instance" , "Feature of herb garden" .)