Pest concealed by a piano (5)
I believe the answer is:
aphid
'pest' is the definition.
(aphid is a type of pest)
'concealed by a piano' is the wordplay.
'concealed' becomes 'hid' ('hide' can be a synonym of 'conceal').
'by' is a charade indicator (letters next to each other).
'piano' becomes 'p' (musical abbreviation).
'a'+'p'='ap'
'hid' put after 'ap' is 'APHID'.
(Other definitions for aphid that I've seen before include "little sucker" , "Plant louse" , "Greenfly etc" , "Horticultural pest" , "Insect feeding on plant juices" .)