A quietly concealed garden pest (5)
I believe the answer is:
aphid
'garden pest' is the definition.
(I've seen this in another clue)
'a quietly concealed' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'concealed' becomes 'hid' ('hide' can be a synonym of 'conceal').
'a'+'p'+'hid'='APHID'
(Other definitions for aphid that I've seen before include "Greenfly etc" , "Blackfly or greenfly" , "Plant-louse" , "plant pest" , "Insect feeding on plant juices" .)