A fish in jelly (4)
I believe the answer is:
agar
'jelly' is the definition.
(gelatinous substance)
'a fish' is the wordplay.
'fish' becomes 'gar' (type of gar).
'a'+'gar'='AGAR'
'in' is the link.
(Other definitions for agar that I've seen before include "contribution to culture?" , "that aids culture" , "Food thickener (derived from seaweed)" , "found in the sea" , "part of culture?" .)