Parasite left on tree, destroying billions (5)
I believe the answer is:
leech
'parasite' is the definition.
(I know that parasite can be written as leech)
'left on tree destroying billions' is the wordplay.
'left on' becomes 'l' (common abbreviation. I am not sure about the 'on' bit.).
'tree' becomes 'beech' (beech is a kind of tree).
'destroying' is a deletion indicator.
'billions' becomes 'b' (abbreviation).
'beech' with 'b' removed is 'eech'.
'l'+'eech'='LEECH'
(Other definitions for leech that I've seen before include "Annelid worm, used in medicine of old" , "Blood-sucking worm or parasitical person" , "Worm - sponger" , "Bloodsucker; early doctor" , "Archaic word for physician" .)