Organism that lives on another (8)
I believe the answer is:
parasite
'organism that lives on another' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for parasite that I've seen before include "It lives off its host" , "host has to put up with" , "Twenty five dn [LEECH]" , "Possibly tick" , "It depends on the host" .)