Smallest meadow, extremely silent (5)
I believe the answer is:
least
'smallest' is the definition.
(synonyms)
'meadow extremely silent' is the wordplay.
'meadow' becomes 'lea' (a lea is a grassy area).
'extremely' suggests removing the centre (only the extremes of the word are used).
'silent' with its centre taken out is 'st'.
'lea'+'st'='LEAST'
(Other definitions for least that I've seen before include "To the lowest degree" , "Bottom" , "Smallest in amount or degree" , "Superlatively little" , "Most insignificant" .)