A fish setting agent (4)
I believe the answer is:
agar
'agent' is the definition.
The definition and answer can be both substances as well as being singular nouns.
Maybe there's an association between them I don't understand?
'a fish' is the wordplay.
'fish' becomes 'gar' (gar is a kind of fish).
'a'+'gar'='AGAR'
'setting' acts as a link.
Can you help me to learn more?
(Other definitions for agar that I've seen before include "found in the sea" , "contribution to culture?" , "used in making soups?" , "Gelatinous substance from seaweed used as a thickener in foods." , "Food thickening agent obtained from seaweed" .)