Flag a recipe containing a thickener (4)
I believe the answer is:
agar
'a thickener' is the definition.
(I've seen this in another clue)
'flag a recipe containing' is the wordplay.
'containing' says the answer is hidden in the clue.
'AGAR' is hidden within 'flag a recipe'.
(Other definitions for agar that I've seen before include "Food-thickening jelly" , "part of culture?" , "Jelly medium" , "Jelly got from seaweeds, used in foods" , "Food thickening agent obtained from seaweed" .)