Decorated new grain outbuilding (9)
I believe the answer is:
garnished
'decorated' is the definition.
(garnishing is a kind of decorating)
'new grain outbuilding' is the wordplay.
'new' is an anagram indicator.
'outbuilding' becomes 'shed' (outbuilding in a garden).
'grain' anagrammed gives 'garni'.
'garni'+'shed'='GARNISHED'
(Other definitions for garnished that I've seen before include "Added parsley, perhaps" , "Decorated" .)