The little pest hid behind a piano (5)
I believe the answer is:
aphid
'the little pest' is the definition.
(insect considered a pest)
'hid behind a piano' is the wordplay.
'behind' 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" , "Greenfly etc" , "insect pest" , "Sap-sucking bug" , "Plant-louse" .)