A fish and seaweed jelly (4)
I believe the answer is:
agar
'seaweed jelly' is the definition.
(I've seen this before)
'a fish' is the wordplay.
'fish' becomes 'gar' (type of gar).
'a'+'gar'='AGAR'
'and' is the link.
(Other definitions for agar that I've seen before include "What comes out of seaweed" , "Gelatinous substance from seaweed used as a thickener in foods." , "contribution to culture?" , "Food-thickening jelly" , "Seaweed derivative" .)