Quietly nod about garden feature (4)
I believe the answer is:
pond
'garden feature' is the definition.
'quietly nod about' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'about' indicates anagramming the letters.
'nod' anagrammed gives 'ond'.
'p'+'ond'='POND'
(Other definitions for pond that I've seen before include "Himalayan country" , "Small area of still water" , "Small body of water" , "Small lake" , "Pool of still water" .)