A fish together with a fish jelly (4-4)
I believe the answer is:
agar-agar
'jelly' is the definition.
(I've seen this before)
'a fish together with a fish' is the wordplay.
'a fish together' becomes 'agar' (I am not sure about this - if you are sure you should believe this answer much more).
'with' is a charade indicator (letters next to each other).
'fish' becomes 'gar' (type of gar).
'agar'+'a'+'gar'='AGAR-AGAR'
(Other definitions for agar-agar that I've seen before include "Jelly from seaweed" , "Food thickener from seaweed" , "Thickener obtained from seaweed" , "Gelatinous thickening for food" .)