Organism that lives in or on another (8)
I believe the answer is:
parasite
'organism that lives in or 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 "louse, say" , "Creature that lives off others" , "Organism growing on or in another" , "It depends on the host" , "host has to put up with" .)