Culture medium (4)
I believe the answer is:
agar
'culture medium' is the definition.
(agar is a kind of culture medium)
This is the entire clue.
(Other definitions for agar that I've seen before include "What comes out of seaweed" , "a newspaper rejected" , "Seaweed jelly" , "Jelly got from seaweeds, used in foods" , "Gelatinous substance from seaweed used as a thickener in foods." .)