Louse lay low behind a piano (5)
I believe the answer is:
aphid
'louse' is the definition.
(aphid is a kind of louse)
'lay low behind a piano' is the wordplay.
'lay low' becomes 'hid' (to lay low is to hide).
'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 "plant pest" , "Greenfly etc" , "Insect feeding on plant juices" , "Small insect like greenfly" , "Small plant-sucking insect" .)