Thickening agent (4)
I believe the answer is:
agar
'thickening agent' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for agar that I've seen before include "contribution to culture?" , "What comes out of seaweed" , "that aids culture" , "part of culture?" , "Gelatinous substance from seaweed used as a thickener in foods." .)